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3 


THE    PRINCIPLES 


BIOLOGY. 


HEEBEET  SPENCEE, 

AUTHOR  OF  "SOCIAL  STATICS,"  "THE  PRINCIPLES  OF  PSYCHOLOGY, 

"  BttSAYB  :  SCIENTIFIC,    POLITICAL,    AND   SPECULATIVE," 
"FIRST   PRINCIPLES,"   ETC. 


VOL.    II. 


NEW    YOEK: 
D.    APPLETON    AND    COMPANY, 

1,  3,  AND   5    BOND    STREET. 

1884. 


Entered,  according  tc  Act  of  Congress,  in  the  year  18«7, 

BY  D   APPLETON  <fc  CO., 

In  tin  Clerk's  Office  of  the  District  Court  of  the  United  State*  for  tie 
Southern  District  of  New  York. 


QM 

SOT 


PREFACE  TO  VOL  1L  I£C"J° 

v.  2^ 


THE  proof  sheets  of  this  volume,  like  those  of  the  last 
volume,  have  been  looked  through  by  Dr.  Hooker  and  Prof. 
Huxley  ;  and  I  have,  as  before,  to  thank  them  for  their 
valuable  criticisms,  and  for  the  trouble  they  have  taken  in 
checking  the  numerous  statements  of  fact  on  which  the  argu- 
ments proceed.  The  consciousness  that  their  many  duties 
render  time  extremely  precious  to  them,  makes  me  feel  ho*v 
heavy  is  my  obligation. 

Part  IV.,  with  which  this  volume  commences,  contains 
numerous  figures.  Nearly  one  half  of  them  are  repetitions, 
mostly  altered  in  scale  and  simplified  in  execution,  of  figures, 
or  parts  of  figure?,  contained  in  the  works  of  various 
Botanists  and  Zoologists.  Among  the  authors  whom  I  have 
laid  under  contribution,  I  may  name  Berkeley,  Carpenter, 
Cuvier,  Green,  Harvey,  Hooker,  Huxley,  Milne-Edwards, 
Ralfs,  Smith.  The  remaining  figures,  numbering  150,  are 
from  original  sketches  and  diagrams. 

The  successive  instalments  which  compose  this  volume, 
were  issued  to  the  Subscribers  at  the  following  dates  :  —  No. 
13  (pp.  1—80)  in  January,  1865;  No.  14  (pp.  81—160)  in 
June,  1865  ;  No.  15  (pp.  161—240)  in  December,  1865;  No. 
16  (pp.  241—320)  in  June,  1866  ;  No.  17  (pp.  321—400)  in 
November,  1866;  and  No.  18  (pp.  401—  566)  in  March,  1867. 

London,  March  23rd,  1867. 


CONTENTS  OF  VOL.  II, 


PART   IV.— MORPHOLOGICAL   DEVELOPMENT. 

Cl'AP.  fAOI 

I. THE    PROBLEMS    CF    MORPHOLOGY                       ...                   ...  3 

II. — THE   MORPHOLOGICAL    COMPOSITION    OF    PLANTS             ...  10 
III. — THE   MORPHOLOGICAL    COMPOSITION    OF    PLANTS,    CON- 
TINUED                 ...              :..               ...               ...  28 

IV. — THE   MORPHOLOGICAL   COMPOSITION   OF   ANIMALS          ...  '77 
V.—  THE    MORPHOLOGICAL   COMPOSITION    OF     ANIMALS,    CON- 
TINUED                      ...                   ...                   ...                   ...  99 

VI. — MORPHOLOGICAL   DIFFERENTIATION    IN    PLANTS               ...  113 

VII. — THE    GENERAL    SHAPES    OF    PLANTS                   ...                   ...  119 

VIII. — THE    SHAPES    OF    BRANCHES             ...                   ...                   ...  130 

IX. — THE    SHAPES    OF    LEAVES                   ...                   ...                   ...  137 

X. THE    SHAPES    OF    FLOWERS                ...                   ...                   ...  140 

XI. THE    SHAPES   OF   VEGETAL   CELLS                       ...                   ...  159 

XII. — CHANGES    OF    SHAPE    OTHERWISE    CAUSED      ...                   ...  162 

XIII. MORPHOLOGICAL   DIFFERENTIATION    IN    ANIMALS            ...  166 

XIV. THE   GENERAL    SHAPES    OF   ANIMALS                 ...                   ...  169 

XV. THE   SHAPES    OF   VERTEBRATE    SKELETONS  ..                     ...  192 

XVI. THE   SHAPES    OF   ANIMAL    CELLS   ...                   ...                   ...  210 

IVII. SUMMARY    OF    MORPHOLOGICAL    DEVELOPMENT                 ...  213 

PART   V— PHYSIOLOGICAL   DEVELOPMENT. 

I. — THE    PROBLEMS    OF    PHYSIOLOGY...                   ...                   ...  221 

II. —DIFFERENTIATIONS    BETWEEN     THE    OUTER   AND    INNER 

TISSUES   OF   PLANTS                    ...                   ...                   ...  226 

Hj. DIFFERENTIATIONS     AMONG     THE     OUTER     TISSUES     OF 

PLANTS                       ...                   ...                   ...                   ...  243 

IV. — DIFFERENTIATIONS      AMONG     THE    DfNER     TISSUES      OF 

PLANTS                        .,,                   ...                   ...                       .  552 


vjii  CONTEXTS. 

PAOC 

CHAP. 

V.— PHYSIOLOGICAL    INTEGRATION    IN    PLANTS    ...                   ...  ^V 

VI.— DIFFERENTIATIONS    BETWEEN     THE   OUTER   AND    INNER 

TISSUES   OF    ANIMALS               ...                  •••                   •«  282 

VII.— DIFFERENTIATIONS     AMONG     THE     OUTER     TISSUES     OF 

ANIMALS                    ...                 •••                  ••'                  •"  2^ 

HI!.— DIFFERENTIATIONS     AMONG     THE     INNER     TISSUES      OF 

ANIMALS                     ...                  ."                   •••                   "•  ^lO 

IX.— PHYSIOLOGICAL   INTEGRATION    IN   ANIMALS...                   ...  365 

X. — SUMMARY   OF   PHYSIOLOGICAL   DEVELOPMENT                    ...  377 

PART  VI.— LAWS  OF  MULTIPLICATION 

I. — THE  FACTORS                   ...                 •••                  •••                  «••  391 

H. A   PRIORI    PRINCIPLE     ...                  ...                   ...                   ...  397 

III. — OBVERSE   A    PRIORI   PRINCIPLE      ...                   ...                   ...  404 

!V. — DIFFICULTIES   OF   INDUCTIVE   VERIFICATION                      ...  412 

V. — ANTAGONISM  BETWEEN   GROWTH  AND   ASEXUAL   GENESIS  419 

VI. — ANTAGONISM   BETWEEN   GROWTH    AND    SEXUAL   GENESIS  428 
VII. — ANTAGONISM    BETWEEN     DEVELOPMENT     AND     GENESIS, 

ASEXUAL   AND   SEXUAL            ...                   ...                   ...  440 

VIII. — ANTAGONISM   BETWEEN    EXPENDITURE   AND    GENESIS  ...  446 

IX. — COINCIDENCE   BETWEEN   HIGH   NUTRITION  AND   GENESIS  454 

X. — SPECIALITIES   OF   THESE   RELATIONS                 ...                   ...  463 

XI. — INTERPRETATION   AND    QUALIFICATION            ...                   ...  470 

XII. — MULTIPLICATION   OF   THE   HUMAN    RACE        ...                   ...  479 

XIII. — HUMAN    POPULATION    IN   THE   FUTURE                                       ,  494 


APPENDICES. 

A..— SUBSTITUTION  OF  AXIAL  FOR  FOLIAR  ORGANS  IN  PLANTS      511 
B. — A     CRITICISM     ON     PROF.     OWEN'S     THEORY     OF    THE    VER- 
TEBRATE  SKELETON  ...  ...  ...       517 

0. — ON     CIRCULATION     AND     THE     FORMATION    OF     WOOD     IN 


PART  IT. 

MORPHOLOGICAL  DEVELOPMENT. 


CHAPTER  I. 

THE   PROBLEMS   OF   MORPHOLOGY. 

§  175.  THE  division  of  Morphology  from  Physiology,  is 
one  which  may  be  tolerably- well  preserved,  so  long  as  we  do 
not  carry  our  inquiries  beyond  the  empirical  generalizations 
of  their  respective  phenomena;  but  it  is  one  which  becomes 
;n  great  measure  nominal,  when  the  phenomena  are  to  be 
rationally  interpreted.  It  would  be  possible,  after  analyzing 
our  Solar  System,  to  set  down  certain  general  truths  respect- 
ing the  sizes  and  distances  of  its  primary  and  secondary 
members,  omitting  all  mention  of  their  motions ;  and  it  would 
be  possible  to  set  down  certain  other  general  truths  respect- 
ing their  motions,  without  specifying  their  dimensions  or 
positions,  further  than  as  greater  or  less,  nearer  or  more  re- 
mote. But  on  seeking  to  account  for  these  general  truths, 
arrived  at  by  induction,  we  find  ourselves  obliged  to  con- 
sider simultaneously  the  relative  sizes  and  places  of  the 
masses,  and  the  relative  amounts  and  directions  of  their 
motions.  Similarly  with  organisms.  Though  we  may  frame 
sundry  comprehensive  propositions  respecting  the  arrange- 
ments of  their  organs,  considered  as  so  many  inert  parts ;  and 
though  we  may  establish  several  wide  conclusions  respecting 
the  separate  and  combined  actions  of  their  organs,  without 
knowing  anything  definite  respecting  the  forms  and  positions 
of  these  organs ;  yet  we  cannot  reach  such  a  rationale  of  the 


4  MORPHOLOGICAL   DEVELOPMENT. 

facts  as  the  hypothesis  of  Evolution  aims  at,  without  contem- 
plating structures  and  functions  in  their  mutual  relations. 
Everywhere  structures  in  great  measure  determine  functions ; 
and  everywhere  functions  are  incessantly  modifying  structures. 
In  Nature,  the  two  are  inseparable  co-operators ;  and  Science 
C;MI  give  no  true  interpretation  of  Nature,  without  keeping 
their  co-operation  constantly  in  view.  An  account  of  organic 
evolution,  in  its  more  special  aspects,  must  be  essentially  an 
account  of  the  inter-actions  of  structures  and  functions,  aa 
perpetually  altered  by  changes  of  conditions. 

Hence,  when  treating  apart  Morphological  Development 
and  Physiological  Development,  all  we  can  do  is  to  direct  our 
attention  mainly  to  the  one  or  to  the  other,  as  the  case  may 
be.  In  dealing  with  the  facts  of  structure,  we  shall  consider 
the  facts  of  function,  only  in  such  general  way  as  is  needful 
to  explain  the  facts  of  structure  ;  and  conversely  when  deal- 
ing with  the  facts  of  function. 

§  176.  The  problems  of  Morphology  fall  into  two  distinct 
classes,  answering  respectively  to  the  two  leading  aspects  of 
Evolution.  In  things  which  evolve  there  go  on  two  processes 
—increase  of  mass  and  increase  of  structure.  Increase  of 
mass  is  primary,  and  in  simple  evolution  takes  place  almost 
alone.  Increase  of  structure  is  secondary,  accompanying  or 
following  increase  of  mass  with  more  or  less  regularity 

•herever  evolution  rises  above  that  form  which  small  inor- 
ganic bodies,  such  as  crystals,  present  to  us.    The  fundamental 

Antagonism  between  Dissolution  and  Evolution  consisting  in 
this,  that  while  the  one  is  an  integration  of  motion  and  dis- 
integration of  matter,  the  other  is  an  integration  of  matter 
and  disintegration  of  motion  ;  and  this  integration  of  matter 
accompanying  disintegration  of  motion,  being   a  necessary 
antecedent  to  the   differentiation  of   the    matter    so   inte 
*;    it  follows  that  questions  concerning  the  mode  in 
uch  the  parts  are  united  into  a  whole,  must  be  dealt  with 


THE  PROBLEMS  OF  MORPHOLOGY.  5 

before  questions  concerning  the  mode  in  which  these  parta 
become  modified.* 

This  is  not  obviously  a  morphological  question.  But  an 
illustration  or  two  will  make  it  manifest,  that  fundamental 
differences  maybe  produced  between  aggregates,  by  differences 
in  the  degrees  of  composition  of  the  increments :  the  ultimate 
units  of  the  increments  being  the  same.  Thus  an  accu- 
mulation of  things  of  a  given  kind  may  be  made  by  add- 
ing one  at  a  time.  Or  the  things  may  be  tied  up  into 
bundles  often,  and  the  tens  placed  together.  Or  the  tens  may 
be  united  into  hundreds,  and  a  pile  of  hundreds  formed.  Such 
unlikenesses  in  the  structures  of  masses,  are  habitually  seen  in 
our  mercantile  transactions.  Articles  which  the  consumer  re- 
cognizes as  single,  the  retailer  keeps  wrapped  up  in  dozens, 
the  wholesaler  sends  the  gross,  and  the  manufacturer  supplies 
in  packages  of  a  hundred  gross — that  is,  they  severally  increase 
their  stocks  by  units  of  simple,  of  compound,  and  of  doubly- 
compound  kinds.  Similarly  result  those  differences  of  mor- 
phological composition  which  we  have  first  to  consider.  An 
organism  consists  of  units.  These  units  may  be  aggregated 
into  a  mass  by  the  addition  of  unit  to  unit.  Or  they  may  be 
united  into  groups,  and  the  groups  joined  together.  Or  these 
groups  of  groups  may  be  so  combined  as  to  form  a  doubly- 
compound  aggregate.  Hence  there  arise  respecting  each 
organic  form,  the  question — is  its  composition  of  the  first, 
Becond,  third,  or  fourth  order  ? — does  it  exhibit  units  of  a 
singly-compounded  kind  only ;  or  are  these  consolidated  into 
units  of  a  doubly-compounded  kind,  or  a  triply-compounded 
kind  ?  And  if  it  displays  double  or  triple  composition,  the 

*  It  seems  needful  here  to  say,  that  allusion  is  made  in  this  paragraph  to  a  pro- 
position respecting  the  ultimate  natures  of  Evolution  and  Dissolution,  which  is 
contained  in  an  essay  on  The  Classification  of  the  Sciences,  published  in  March, 
18G4.  When  the  opportunity  comes,  I  hope  to  make  the  definition  there  arrived 
at,  the  basis  of  a  re-organization  of  the  second  part  of  First  Principles  :  giving  to 
*hat  work  a  higher  development,  and  a  greater  cohesion,  than  it  at  present  pc*>« 


6  MORPHOLOGICAL    DEVELOPMENT. 

homologies  of  its  different  parts  become  problems.  Undei 
the  disguises  induced  by  the  consolidation  of  primary,  second- 
ary, and  tertiary  units,  it  has  to  be  ascertained  which  answer 
to  which,  in  their  degrees  of  composition. 

Such  questions  are  more  intricate  than  they  at  first  appear ; 
since,  besides  the  obscurities  caused  by  progressive  integration, 
and  those  due  to  accompanying  modifications  of  form,  further 
obscurities  result  from  the  variable  growths  of  units  of  the 
different  orders.  Just  as  an  army  may  be  augmented  by  re- 
cruiting in  each  company,  without  increasing  the  number  of 
companies ;  or  may  be  augmented  by  making  up  the  full 
complement  of  companies  in  each  regiment,  while  the 
number  of  regiments  remains  the  same ;  or  may  be  aug- 
mented by  putting  more  regiments  into  each  division,  other 
things  being  unchanged ;  or  may  be  augmented  by  adding  to 
the  number  of  its  divisions  without  altering  the  components  of 
each  division ;  or  may  be  augmented  by  two  or  three  of  these 
processes  at  once ;  so,  in  organisms,  increase  of  mass  may  be 
due  to  growth  in  units  of  the  first  order,  or  in  those  of  the 
second  order,  or  in  those  of  still  higher  orders ;  or  it  may  be 
due  to  simultaneous  growth  in  units  of  several  orders. 
And  this  last  mode  of  integration  being  the  general  mode, 
puts  difficulties  in  the  way  of  analysis.  Just  as  the  structure 
of  an  army  would  be  made  less  easy  to  understand,  if  com- 
panies often  outgrew  regiments,  or  regiments  became  larger 
than  brigades  ;  so  these  questions  of  morphological  composi- 
tion, are  complicated  by  the  indeterminate  sizes  of  the  units 
of  each  kind — relatively-simple  units  frequently  becoming 
far  more  bulky  than  relatively-compound  units. 

§  177.  The  morphological  problems  of  the  second  class, 
are  those  having  for  their  subject-matter  the  changes  of  shape 
that  accompany  changes  of  aggregation.  The  most  general 
questions  respecting  the  structure  of  an  organism,  having  been 
answered  when  it  is  ascertained  of  what  units  it  is  composed  as 
a  whole,  and  in  its  several  parts ;  there  come  the  more  special 


THE  PROBLEMS  OF  MORPHOLOGY.  / 

questions  concerning  its  form — form  in  the  ordinary  sense. 
After  the  contrasts  caused  by  variations  in  the  process  of 
integration,  we  have  to  consider  the  contrasts  caused  by 
variations  in  the  process  of  differentiation.  To  speak  speci- 
fically— the  shape  of  the  organism  as  a  whole,  irrespect- 
ive of  its  composition,  has  to  be  accounted  for.  Reasons 
have  to  be  found  for  the  unlikeness  between  its  general  out- 
lines and  the  general  outlines  of  allied  organisms.  And  there 
have  to  be  answered  kindred  inquiries  respecting  the  propor- 
tions of  its  component  parts : — Why,  among  such  of  these  as 
are  homologous  with  one  another,  have  there  arisen  the 
differences  that  exist  ?  And  how  have  there  been  produced 
the  contrasts  between  them  and  the  homologous  parts  of  or- 
ganisms of  the  same  type  ? 

Yery  numerous  are  the  heterogeneities  of  form  that  present 
themselves  for  interpretation  under  these  heads.  The  ultimate 
morphological  units  combined  in  any  group,  may  be  differ- 
entiated individually,  or  collectively,  or  both  :  each  of  them 
may  undergo  changes  of  shape  ;  or  some  of  them  may  be 
changed  and  others  not ;  or  the  group  may  be  rendered  mul- 
tiform by  the  greater  growth  of  some  of  its  units  than  of 
others.  Similarly  with  the  compound  units,  arising  by  union 
of  these  simple  units.  Aggregates  of  the  second  order  may 
be  made  relatively  complex  in  form,  by  inequalities  in  the 
rates  of  multiplication  of  their  component  units  in  diverse 
directions  ;  and  among  a  number  of  such  aggregates,  numer- 
ous unlikenesses  may  be  constituted  by  differences  in  their 
degrees  of  growth,  and  by  differences  in  their  modes  of 
growth.  Manifestly,  at  each  higher  stage  of  composition,  the 
possible  sources  of  divergence  are  multiplied  still  further. 

That  facts  of  this  order  can  be  accounted  for  in  detail,  is 
not  to  be  expected — the  data  are  wanting.  All  that  we  may 
hope  to  do,  is  to  ascertain  their  general  laws.  How  this  is  to 
be  attempted  we  will  now  consider. 

§  178.  The  task  before  us  is  to  trace  throughout  these 


8  MORPHOLOGICAL    DEVELOPMENT. 

phenomena  the  process  of  evolution ;  and  to  show  how,  as 
displayed  in  them,  it  conforms  to  those  first  principles  which 
evolution  in  general  conforms  to.  Two  sets  of  factors  have 
to  be  taken  into  account.  Let  us  look  at  them. 

The  factors  of  the  first  class  are  those  which  tend  directly 
to  change  an  organic  aggregate,  in  common  with  every  other 
aggregate,  from  that  more  simple  form  which  is  not  in  equi- 
librium with  incident  forces,  to  that  more  complex  form  which 
is  in  equilibrium  with  them.  We  have  to  mark  how,  in  corre- 
spondence with  the  universal  law  that  the  uniform  lapses  into 
the  multiform,  and  the  less  multiform  into  the  more  multi- 
form, the  parts  of  each  organism  are  ever  becoming  further 
differentiated  ;  and  we  have  to  trace  the  varying  relations  to 
incident  forces,  by  which  further  differentiations  are  entailed. 
We  have  to  observe,  too,  how  each  primary  modification  of 
structure,  induced  by  an  altered  distribution  offerees,  becomes 
a  parent  of  secondary  modifications — how,  through  the  neces- 
sary multiplication  of  effects,  change  of  form  in  one  part  brings 
about  changes  of  form  in  other  parts.  And  then  we  have  also 
to  note  the  metamorphoses  constantly  being  induced  by  the 
process  of  segregation — by  the  gradual  union  of  like  parts 
exposed  to  like  forces,  and  the  gradual  separation  of  like  parts 
exposed  to  uulike  forces.  The  factors  of  the  second 

class  which  we  have  to  keep  in  view  throughout  our  interpret- 
ations, are  the  formative  tendencies  of  organisms  themselves 
— the  proclivities  inherited  by  them  from  antecedent  organ- 
isms, and  which  past  processes  of  evolution  have  bequeathed. 
We  have  seen  it  to  be  a  necessary  inference  from  various  orders 
of  facts  (§§  65,  84,  97.)  that  organisms  are  built  up  of  certain 
highly-complex  molecules,  which  we  distinguished  as  physio- 
logical units— each  kind  of  organism  being  built  up  of  phy- 
siological units  peculiar  to  itself.  We  found  ourselves  obliged 
to  recognize  in  these  physiological  units,  powers  of  arranging 
themselves  into  the  forms  of  the  organisms  to  which  they  be- 
long ;  analogous  to  the  powers  which  the  molecules  of  inor- 
ganic substances  have  of  aggregating  into  specific  crystalline 


THE   PROBLEMS   OF    MORPHOLOGY.  9 

forms.  "We  have  consequently  to  regard  this  polarity  of  the 
physiological  units,  as  producing,  during  the  development  of 
any  organism,  a  combination  of  internal  forces  that  expend 
themselves  in  working  out  a  structure  in  equilibrium  with 
the  forces  to  which  ancestral  organisms  were  exposed ;  but 
not  in  equilibrium  with  the  forces  to  which  the  existing  organ- 
ism is  exposed,  if  the  environment  has  been  changed.  Hence 
the  problem  in  all  cases,  is,  to  ascertain  the  resultant  of  inter- 
nal organizing  forces,  tending  to  reproduce  the  ancestral  form, 
and  external  modifying  forces,  tending  to  cause  deviations  from 
that  form.  Moreover,  we  have  to  take  into  account, 

not  only  the  characters  of  immediately-preceding  ancestors, 
but  also  those  of  their  ancestors,  and  ancestors  of  all  degrees  of 
remoteness.  Setting  out  with  rudimentary  types,  we  have 
to  consider  how,  in  each  successive  stage  of  evolution,  the 
structures  acquired  during  previous  stages,  have  been  ob- 
scured by  further  integrations  and  further  differentiations ; 
or,  conversely,  how  the  lineaments  of  primitive  organisms 
have  all  along  continued  to  manifest  themselves  under  the 
superposed  modifications. 

§  179.  Two  ways  of  carrying  on  the  inquiry  suggest 
themselves.  ~VVe  may  go  through  the  several  great  groups 
of  organisms,  with  the  view  of  reaching,  by  comparison 
of  parts,  certain  general  truths  respecting  the  homologies, 
the  forms,  and  the  relations  of  their  parts ;  and  then,  having 
dealt  with  the  phenomena  inductively,  may  retrace  our  steps 
with  the  view  of  deductively  interpreting  the  general  truths 
reached.  Or,  instead  of  thus  separating  the  two  inves- 
tigations, we  may  carry  them  on  hand  in  hand — first 
establishing  each  general  truth  empirically,  and  then  pro- 
ceeding to  the  rationale  of  it.  This  last  method  will,  I 
think,  conduce  to  both  brevity  and  clearness.  Let  us  now 
thus  deal  with  the  first  class  of  morphological  problems. 


CHAPTER  H. 

THE  MORPHOLOGICAL   COMPOSITION   OF  PLANTS. 

§  180.  Evolution  implies  insensible  modifications  and 
gradual  transitions,  which  render  definition  difficult — which 
make  it  impossible  to  separate  absolutely  the  phases  of  or- 
ganization from  one  another.  And  this  indefiniteness  of 
distinction,  to  be  expected  a  priori,  we  are  compelled  to  re- 
cognize a  posteriori,  the  moment  we  begin  to  group  morpho- 
logical phenomena  into  general  propositions.  Thus,  on  in- 
quiring what  is  the  morphological  unit,  whether  of  plants  or 
of  animals,  we  find  that  the  facts  refuse  to  be  included  in  any 
rigid  formula.  The  doctrine  that  all  organisms  are  built  up 
of  cells,  or  that  cells  are  the  elements  out  of  which  every 
tissue  is  developed,  is  but  approximately  true.  There  are 
living  forms  of  which  cellular  structure  cannot  be  asserted ; 
and  in  living  forms  that  are  for  the  most  part  cellular,  there 
are  nevertheless  certain  portions  which  are  not  produced  by 
the  metamorphosis  of  cells.  Supposing  that  clay  were  the  only 
material  available  for  building,  the  proposition  that  all  houses 
are  built  of  bricks,  would  bear  about  the  same  relation  to  the 
truth,  as  does  the  proposition  that  all  organisms  are  composed 
of  cells.  This  generalization  respecting  houses  would  be 
open  to  two  criticisms :  first,  that  certain  houses  of  a  primi- 
tive kind  are  formed,  not  out  of  bricks,  but  out  of  unmoulded 
clay ;  and  second,  that  though  other  houses  consist  mainly  of 
bricks,  yet  their  chimney-pots,  drain-pipes,  and  ridge-tiles 


THE   MORPHOLOGICAL  COMPOSITION    OF   P^&NTS.  11 

do  not  result  from  combination  or  metamorphosis  of  bricks, 
but  are  made  directly  out  of  the  original  clay.  And  of  like 
natures  are  the  criticisms  which  must  be  passed  on.  the 
generalization,  that  cells  are  the  morphological  units  of  or- 
ganisms. To  continue  the  simile,  the  truth  turns  out  to 
be,  that  the  primitive  clay  or  protoplasm  out  of  which 
organisms  are  built,  may  be  moulded  either  directly,  or 
with  various  degrees  of  indirectness,  into  organic  struc- 
tures. The  physiological  units  which  we  are  obliged  to  as- 
sume as  the  components  of  this  protoplasm,  must,  as  we  have 
seen,  be  the  possessors  of  those  complex  polarities  which  re- 
sult in  the  structural  arrangements  of  the  organism.  The 
assumption  of  such  structural  arrangements  may  go  on,  and, 
in  many  cases,  does  go  on,  by  the  shortest  route ;  without  the 
passage  through  what  we  call  metamorphoses.  But  where 
such  structural  arrangements  are  reached  by  a  circuitous 
route,  the  first  stage  is  the  formation  of  these  small  aggre- 
gates, which,  under  the  name  of  cells,  are  currently  regarded 
as  morphological  units. 

The  rationale  of  these  truths  appears  to  be  furnished  by  the 
hypothesis  of  evolution.  We  set  out  with  molecules  one 
degree  higher  in  complexity  than  those  molecules  of  nitro- 
genous colloidal  substance  into  which  organic  matter  is 
resolvable ;  and  we  regard  these  somewhat  more  complex  mo- 
lecules as  having  the  implied  greater  instability,  greater  sen- 
sitiveness to  surrounding  influences,  and  consequent  greater 
mobility  of  form.  Such  being  the  primitive  physiological 
units,  organic  evolution  must  begin  with  the  formation  of  a 
minute  aggregate  of  them — an  aggregate  showing  vitality 
only  by  a  higher  degree  of  that  readiness  to  change  its  form 
of  aggregation,  which  colloidal  matter  in  general  displays ; 
and  by  its  ability  to  unite  the  nitrogenous  molecules  it  meets 
with,  into  complex  molecules  like  those  of  which  it  is  com- 
posed. Obviously,  the  earliest  forms  must  have  been  minute ; 
since,  in  the  absence  of  any  but  diffused  organic  matter,  no 
form  but  a  minute  one  could  find  nutriment.  Obviously,  too> 


12  MORPHOLOGICAL    DEVELOPMENT. 

it  must  have  been  structurelesss ;  since,  as  differentiations  are 
producible  only  by  the  unlike  actions  of  incident  forces,  there 
could  have  been  no  differentiations  before  such  forces  had  had 
time  to  work.  Hence,  distinctions  of  parts  like  those  required 
to  constitute  a  cell,  were  necessarily  absent  at  first.  And  we 
need  not  therefore  be  surprised  to  find,  as  we  do  find,  specks 
of  protoplasm  manifesting  life,  and  yet  showing  no  signs  of 
organization.  A  further  stage  of  evolution  is 

reached,  when  the  very  imperfectly  integrated  molecules  form- 
ing one  of  these  minute  aggregates,  become  more  coherent ; 
at  the  same  time  as  they  pass  into  a  state  of  heterogeneity, 
gradually  increasing  in  its  definiteness.  That  is  to  say,  we 
may  look  for  the  assumption  by  them,  of  some  distinctions  of 
parts,  such  as  we  find  in  cells,  and  in  what  are  called  uni- 
cellular organisms.  They  cannot  retain  their  primordial  uni- 
formity ;  and  while  in  a  few  cases  they  may  depart  from  it 
but  slightly,  they  will,  in  the  great  majority  of  cases,  acquire 
a  very  decided  multiformity — there  will  result  the  compara- 
tively integrated  and  comparatively  differentiated  Protophyta 
and  Protozoa.  The  production  of  minute  aggregates 

of  physiological  units,  being  the  first  step ;  and  the  passage  of 
such  minute  aggregates  into  more  consolidated  and  more 
complex  forms,  being  the  second  step ;  it  must  naturally  hap- 
pen that  all  higher  organic  types,  subsequently  arising  by 
further  integrations  and  differentiations,  will  everywhere  bear 
the  impress  of  this  earliest  phase  of  evolution.  From  the 
law  of  heredity,  considered  as  extending  to  the  entire  succes- 
sion of  living  things  during  the  Earth's  past  history,  it 
follows,  that  since  the  formation  of  these  small,  simple  organ- 
isms, must  have  preceded  the  formation  of  larger  and  more 
complex  organisms,  the  larger  and  more  complex  organisms 
must  inherit  their  essential  characters.  We  may  anticipate 
that  the  multiplication  and  combination  of  these  minute 
aggregates  or  cells,  will  be  conspicuous  in  the  early  develop- 
mental stages  of  plants  and  animals  ;  and  that  through- 
out all  subsequent  stages,  cell-production  and  cell-differen 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  13 

tiation  will  be  dominant  characteristics.  The  physiological 
units  peculiar  to  each  higher  species,  will,  speaking  generally, 
pass  through  this  form  of  aggregation  on  their  way  towards 
the  final  arrangement  they  are  to  assume ;  because  those 
primordial  physiological  units  from  which  they  are  remotely 
descended,  aggregated  into  this  form.  And  yet,  just  as  in 
other  cases  we  found  reasons  for  inferring  (§  131),  that  the 
traits  of  ancestral  organization  may,  under  certain  conditions, 
be  partially  or  wholly  obliterated,  and  the  ultimate  structure 
assumed  without  passing  through  them ;  so,  here,  it  is  to  be 
inferred  that  the  process  of  cell- formation  may,  in  some  cases, 
be  passed  over.  Thus  the  hypothesis  of  evolution 

prepares  us  for  those  two  radical  modifications  of  the  cell- 
doctrine,  which  the  facts  oblige  us  to  make.  It  leads  us  to 
expect  that  as  structureless  portions  of  protoplasm  must  have 
preceded  cells  in  the  process  of  general  evolution ;  so,  in  the 
special  evolution  of  each  higher  organism,  there  will  be 
an  habitual  production  of  cells  out  of  structureless  blastema. 
And  it  leads  us  to  expect  that  though,  generally,  the  physiolo- 
gical units  composing  a  structureless  blastema,  will  display 
their  inherited  proclivities  by  cell-development  and  meta- 
morphosis ;  there  will  nevertheless  occur  cases  in  which  the 
tissue  to  be  formed,  is  formed  by  direct  transformation  of  the 
blastema. 

Interpreting  the  facts  in  this  manner,  we  may  recognize 
that  large  amount  of  truth  which  the  cell-doctrine  contains, 
without  committing  ourselves  to  the  errors  involved  by  the 
sweeping  assertion  of  it.  We  are  enabled  to  understand  how 
it  happens  that  organic  structures  are  usually  cellular  in  their 
composition,  at  the  same  time  that  they  are  not  universally 
so.  We  are  shown  that  while  we  may  properly  continue  to 
regard  the  cell  as  the  morphological  unit,  we  must  constantly 
bear  in  mind  that  it  is  such,  only  in  a  greatly-qualified  sense.* 

*  Let  me  here  refer  those  who  are  interested  in  this  question,  to  Prof.  Hux- 
ley's criticism  on  the  cell-doctrine,  published  in  the  Hedico-Chirurgical  Seview 
in  1853. 


11 


MORPHOLOGICAL    DEVELOPMENT. 


§  181.  These  aggregates  of  the  lowest  order,  each  formed  of 
physiological  units  united  into  a  group  that  is  structurely 
single,  and  cannot  be  divided  without  destruction  of  its 
individuality,  may,  as  above  implied,  exist  as  independent 
organisms.  The  assumption  to  which  we  are  committed  by 
the  hypothesis  of  evolution,  that  such  so-called  uni-cellular 
plants,  were  at  first  the  only  kinds  of  plants,  is  in  harmony 
with  the  fact  that  habitats  not  occupied  by  plants  of  higher 
orders,  commonly  contain  these  protophytes  in  great  abund- 
anco  and  great  variety.  The  various  species  of  Protococcus, 
of  Detmidiacece,  and  Diatomacece,  supply  examples  of  morpho- 
logical units  living  and  propagating  separately,  under  nu- 
merous modifications  of  form  and  structure.  Figures  1,  2, 
and  3,  represent  a  few  of  the  commonest  types. 


Mostly,  simple  plants  are  too  small  to  be  individually 
visible  without  the  microscope.  But,  in  some  cases,  these 
vegetal  aggregates  of  the  first  order,  grow  to  appreciable 
sizes.  In  the  mycelium  of  some  fungi,  we  have  single  cells 
developed  into  long  branched  filaments,  or  ramified  tubules, 
that  are  of  considerable  lengths.  An  analogous  structure 
characterizes  certain  tribes  of  Alga,  of  which  Codium  adhcerem, 
Fig.  4,  may  serve  as  an  example.  In  Hydrogastrum,  an- 
other alga,  Fig.  5,  we  have  a  structure  which  is  described  as 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  15 

simulating  a  perfect  plant,  with  root,  stem,  bud,  and  fruit, 
all  produced  by  the  branching  of  a  single  cell.  And 
among  fungi,  the  genus  Botrytis,  Fig.  6,  furnishes  an  illus- 
tration of  allied  kind.  Here,  though  the  size  attained  is 
much  greater  than  that  of  many  organisms  which  are  mor- 
phologically compound,  we  are  compelled  to  consider  the 
morphological  composition  as  simple ;  since  the  whole  can  no 
more  be  separated  into  minor  wholes,  than  can  the  branched 
vascular  system  of  an  animal.  In  these  cases,  we  have  con- 
siderable bulk  attained,  not  by  a  number  of  aggregates  of 
the  first  order  being  united  into  an  aggregate  of  the  second 
order ;  but  by  the  continuous  growth  of  an  aggregate  of  the 
first  order. 

§  182.  The  transition  to  higher  forms  begins  in  a  very 
unobtrusive  manner.  Among  these  aggregates  of  the  first 
order,  an  approach  towards  that  union  by  which  aggregates 
of  the  second  order  are  produced,  is  indicated  by  mere  juxta- 
position. Protophytes  multiply  rapidly;  and  their  rapid 
multiplication  sometimes  causes  crowding.  "When,  instead 
of  floating  free  in  the  water,  they  form  a  thin  film  on  a  moist 
surface,  or  are  imbedded  in  a  common  matrix  of  mucus  ;  the 
mechanical  obstacles  to  dispersion  result  in  a  kind  of  feeble 
integration,  vaguely  shadowing  forth  a  combined  group. 
Somewhat  more  definite  combination  is  shown  us  by  such 
plants  as  Palmella  botryoides.  Here  the  members  of  a  family 
of  cells,  arising  by  the  spontaneous  fission  of  a  parent-cell, 
remain  united  by  slender  threads  of  that  jelly-like  substance 
which  envelops  their  surfaces.  In  some  Diatomacece,  several 
individuals,  instead  of  completely  separating,  hold  together 
by  their  angles  ;  and  in  other  Diatomacece,  as  the  Bacillaria, 
a  variable  number  of  units  cohere  so  slightly,  that  they  are 
continually  moving  in  relation  to  one  another. 

This  formation  of  aggregates  of  the  second  order,  faintly 
indicated  in  feeble  and  variable  unions  like  the  above,  may 
be  traced  through  phases  of  increasing  permanence  and  de« 


16  MORPHOLOGICAL    DEVELOPMENT. 

finiteness,  as  well  as  increasing  extent.  In  the  yeast-plant, 
Fig.  7,  we  have  cells  which  may  exist  singly,  or  joined  into 
groups  of  several ;  and  which  have  their  shapes  scarcely  at 
all  modified  by  their  connexion.  Among  the  Desmidiacece,  it 
happens  in  many  cases,  that  the  two  individuals  produced  by 
division  of  a  parent-individual,  part  as  soon  as  they  are  fully 
formed  ;  but  in  other  cases,  instead  of  parting  they  compose 
a  group  of  two.  Allied  kinds  show  us  how,  by  subsequent 
fissions  of  the  adherent  individuals  and  their  progeny,  there 
result  longer  groups ;  and  in  some  species,  a  continuous  thread 
of  them  is  thus  produced.  Figs.  8,  9,  10,  11,  exhibit  these 

JTi 


several  stages.  Instead  of  linear  aggregation,  some  of  the 
Dcsmidiaccoe  illustrate  central  aggregation ;  as  shown  in 
Figs.  12,  13,  14,  15.  Other  instances  of  central  aggrega- 
tion are  furnished  by  such  protophytes  as  the  Gonium  pector- 
ale,  Fig.  16  (a  being  the  front  view,  and  b  the  edge  view), 
and  the  Sarcina  ventriculi,  Fig.  17.  Further,  we  have  that 
spherical  mode  of  aggregation  of  which  the  Volcox  globator 
furnishes  a  familiar  instance. 

Thus  far,  however,  the  individuality  of  the  secondary  ag- 
gregate is  feebly  pronounced :  not  simply  in  the  sense  that 
it  is  small ;  but  also  in  the  sense  that  the  individualities  of  the 
primary  aggregates  are  very  little  subordinated.  But  on 
seeking  further,  we  find  transitions  towards  forms  in  which 
the  compound  individuality  is  more  dominant,  while  the  sim- 
ple individualities  are  more  obscured.  Obscuration 
of  one  kind,  accompanies  mere  increase  of  size  in  the  second- 
ary aggregate :  in  proportion  to  the  greater  number  of  the 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS. 


1? 


morphological  units  held  together  in  one  mass,  becomes  their 
relative  insignificance  as  individuals.  We  see  this  in  the 
irregularly-spreading  lichens  that  form  patches  on  rocks ; 
and  in  such  creeping  fungi  as  grow  in  films  or  laminso  on 
decaying  wood  and  the  bark  of  trees.  In  these  cases,  how- 
ever, the  integration  of  the  component  cells  is  of  an  almost 
mechanical  kind.  The  aggregate  of  them  is  scarcely  more 
individuated  than  a  lump  of  inorganic  matter :  as  witness  the 
way  in  which  the  lichen  extends  its  curved  edges  in  this  or 
that  direction,  as  the  surface  favours  ;  or  the  way  in  which 
the  fungus  grows  round  and  imbeds  the  shoots  and  leaves  that 
lie  in  its  way,  just  as  so  much  plastic  clay  might  do;  Though 
here,  in  the  augmentation  of  mass,  we  see  a  progress  towards 
the  evolution  of  a  higher  type ;  we  have  as  yet  none  of  that 
definiteness  required  to  constitute  a  compound  unit,  or  true 
aggregate  of  the  second  order.  Another  kind  of 

obscuration  of  the  morphological  units,  is  brought  about  by 
their  more  complete  coalescence  into  the  form  of  some  struc- 
ture made  by  their  union.  This  is  well  exemplified  among 
tl  e  Confervas,  and  their  allies.  In  Fig.  18,  there  are  re- 


presented  the  stages  of  a  growing  Mougeotia  genuflexa,  in 
which  this  merging  of  the  simple  individualities  into  the 
compound  individuality,  is  shown  in  the  history  of  a  single 
plant ;  and  in  Figs.  19,  20,  21,  22,  23,  are  represented  a  series 
of  species  from  this  group,  and  that  of  Cladophora,  in  which 
we  see  a  progressing  integration.  "While  in  the  lower  types, 
the  primitive  spheroidal  forms  of  the  cells  are  scarcely 
Voi,,  II.  2 


1$  MORPHOLOGICAL   DEVELOPMENT. 

altered  ;  in  the  higher  types,  the  cells  are  so  fused  together 
as  to  constitute  cylinders  divided  by  septa.  Here,  how- 
ever, the  indefiniteness  is  still  great :  there  are  no  specific 
limits  to  the  length  of  any  thread  thus  produced  ;  and  none 
of  that  differentiation  of  parts  required  to  give  a  decided  in- 
dividuality to  the  whole. 

To  constitute  something  like  a  true  aggregate  of  the 
second  order,  capable  of  serving  as  a  compound  unit,  that 
may  be  combined  with  others  like  itself  into  still  higher 
aggregates,  there  must  exist  both  mass  and  definiteness. 

§  183.  An  approach  towards  plants  which  unite  these  cha- 
racters, may  be   traced  in   such   forms  as  Bangia  ciliaris, 
Fig.  24.     The  multiplication  of  cells  here  takes  place,  not  in 
a  longitudinal  direction  only,  but  also  in  a 
«  transverse   direction ;    and   the    transverse 

multiplication  being  greater  towards  the 
middle  of  the  frond,  there  results  a  differ- 
ence between  the  middle  and  the  two  ex- 
tremities— a  character  which,  in  a  feeble 
way,  unites  all  the  parts  into  a  whole.  Even 
this  slight  individuation  is,  however,  very 
indefinitely  marked ;  since,  as  shown  by 
the  figures,  the  lateral  multiplication  of  cells 
I  does  not  go  on  in  a  precise  manner. 

From  some  such  type  as  this  there  appear 
I  to  arise,  by  slight  differences  in  the  modes  of 
growth,  two  closely-allied  groups  of  plants, 
having  individualities  somewhat  more  pro- 
nounced. If,  while  the  cells  multiply  lon- 
gitudinally, their  lateral  multiplication  goes  on  in  one  direc- 
tion only,  there  results  a  flat  surface,  as  in  Ulva  linza,  Fig. 
25  ;  or  where  the  lateral  multiplication  is  less  uniform  in  its 
rate,  in  types  like  Fig.  26.  But  where  the  lateral  multipli- 
cation occurs  in  two  directions  transverse  to  one  another, 
u  hollow  frond  may  be  produced — sometimes  irregularly 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS. 


19 


fcpheroidal,  and  sometimes  irregularly  tubular ;  as  in  Enter -o- 
morpha  intestinalis,  Fig.  27.  And  occasionally,  as  in  Entero- 
morpha  comprcssa,  Fig.  28,  this  tubular  frond  becomes  branched. 
Figs.  29  and  30  are  magnified  portions  of  such  fronds  ;  show- 


ing  the  simple  cellular  aggregation  which  allies  them  with 
the  preceding  forms. 

In  the  common  Fuci  of  our  coasts,  other  and  somewhat 
higher  stages  of  this  integration  are  displayed.  We  have 
fronds  preserving  something  like  constant  breadths ;  and 
dividing  dichotomously  with  approximate  regularity.  Though 
the  sub-divisions  so  produced,  are  not  to  be  regarded  at  all  as 
separate  fronds,  but  only  as  extensions  of  one  frond,  they 
foreshadow  a  higher  degree  of  composition  ;  and  by  the  com- 
paratively methodic  way  in  which  they  are  united,  give  to 
the  aggregate  a  more  definite,  as  well  as  a  more  complex,  in- 
dividuality. Many  of  the  higher  lichens  exhibit  an 
analogous  advance.  While  in  the  lowest  lichens,  the  different 
parts  of  the  thallus  are  held  together  only  by  being  all 
attached  to  the  supporting  surface,  in  the  higher  lichens  the 
thallus  is  so  far  integrated  that  it  can  support  itself  by 
attachment  to  such  surface  at  one  point  onlv.  And  then,  in 
still  more  developed  kinds,  we  find  the  thallus  assuming  a 
dichotomously-branched  form,  and  so  gaining  a  more  specific 
character  as  well  as  greater  size. 


20  MORPHOLOGICAL    DEVELOPMENT 

Where,  as  in  types  like  these,  the  morphological  units 
show  an  inherent  tendency  to  arrange  themselves  in  a  man- 
ner that  is  so  far  constant  as  to  give  characteristic  propor- 
tions, we  may  say  that  there  is  a  recognizable  compound  in- 
dividuality. Considering  the  Thallogens  that  grow  in  this 
way,  apart  from  their  kinships,  and  wholly  with  reference  to 
their  morphological  composition,  we  might  not  inaptly  de- 
scribe them  as  pseudo-foliar. 

§  184.  Another  mode  in  which  aggregation  is  so  carried 
on  as  to  produce  a  compound  individuality  of  considerable 
defmiteness,  is  variously  displayed  among  other  families  of 
A/ij(c.  When  the  cells,  instead  of  multiplying  longitudin- 
ally alone,  and  instead  of  all  multiplying  laterally  as  well  as 
longitudinally,  multiply  laterally  only  at  particular  places  ; 
they  produce  a  branched  structure. 

Indications  of  this  mode  of  aggregation  occur  among  the 
Confercw  and  simple  plants  akin  to  them,  as  shown  in  Figs. 
22,  23.  Though,  in  some  of  the  more  developed  Algce  which 
exhibit  the  ramified  arrangement  in  a  higher  degree,  the 
component  cells  are,  like  those  of  the  lower  Algce,  united  to- 
gether end  to  end,  in  such  way  as  but  little  to  obscure  their 
separate  forms,  as  in  Cladophora  Hutchinsice,  Fig.  31 ;  they 


. 

nevertheless  evince  greater  subordination  to  the  whole  of 
which  they  are  parts,  by  arranging  themselves  more  method- 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLAKTS.  21 

ically.  Still  further  pronounced  becomes  the  compound 
individuality,  when,  while  the  component  cells  of  the 
branches  unite  completely  into  jointed  cylinders,  the  com- 
ponent cells  of  the  stem  begin  to  multiply  laterally,  so  as  to 
form  an  axis  distinguished  by  its  relative  thickness  and  com- 
plexity. Such  types  of  structures  are  indicated  by  Figs.  32, 
33 — figures  representing  small  portions  of  plants  which  are 
quite  tree-like  in  their  entire  outlines.  On  examining 
Figs.  34,  35,  36,  which  show  the  structures  of  the  stems  in 
these  types,  it  will  be  seen,  too,  that  the  component  cells  in 
becoming  more  coherent,  have  undergone  changes  of  form 
which  obscure  their  individualities  more  than  before :  not 
only  are  they  much  elongated,  but  they  are  so  compressed  as 
to  be  prismatic  rather  than  cylindrical.  This  structure,  be- 
sides displaying  integration  of  the  morphological  units  car- 
ried on  in  two  directions  instead  of  one;  and  besides  displaying 
this  higher  integration  in  the  greater  merging  of  the  indi- 
vidualities of  the  morphological  units  in  the  general  individu- 
ality ;  also  displays  it  in  the  more  pronounced  subordination 
of  the  branches  and  branchlets  to  the  main  stem.  This  differ- 
entiation and  consolidation  of  the  stem,  brings  all  the  second- 
ary growths  into  more  marked  dependence ;  and  so  renders 
the  individuality  of  the  aggregate  more  decided. 

We  might  not  inappropriately  call  this  type  of  structure 
pseud-axial.  It  simulates  that  of  the  higher  plants  in  cer- 
tain leading  characters.  We  see  in  it  a  primary  axis  along 
which  development  may  continue  indefinitely,  and  from 
which  there  bud  out,  laterally,  secondary  axes  of  like  na- 
ture, bearing  like  tertiary  axes ;  and  this  is  the  mode  of 
growth  with  which  Phaenogams  make  us  familiar.  But  the 
resemblance  goes  no  further  ;  for  these  pseud-axes  are  devoid 
of  those  lateral  appendages — those  leaves  or  foliar  organs — 
which  true  axes  bear,  and  the  bearing  of  which  ordinarily 
constitutes  them  true  axes. 

§  185.  Some  of  the  larger  Algce  supply  examples  of  an 


22  MOUPHOLOGICAL   DEVELOPMENT. 

integration  still  more  advanced:  not  simply  inasmuch  as 
they  unite  much  greater  numbers  of  morphological  units 
into  continuous  masses;  but  also  inasmuch  as  they  com- 
bine the  pseudo-foliar  structure  with  the  pseud-axial  struc- 
ture. Our  own  shores  furnish  an  instance  of  this  in  the 
common  Laminaria ;  and  certain  gigantic  Fuel  of  the 
Antartic  seas,  supply  yet  better  instances.  In  some  of 
these,  the  germ  develops  a  very  long  slender  stem,  which 
eventually  expands  into  a  large  bladder-like  or  cylindrical 
air-vessel;  and  from  the  surface  of  this  there  grow  out 
numerous  leaf-shaped  expansions.  Another  kind,  Lessonia 
fascescens,  Fig.  37,  shows  us  a  massive  stem  growing  up 
through  water  many  feet  deep — a  stem  which, 
bifurcating  as  it  approaches  the  surface,  flat- 
tens out  the  ends  of  its  subdivisions  into  fronds 
like  ribands.  These,  however,  are  not  true 
foliar  appendages,  since  they  are  merely  ex- 
panded continuations  of  the  stem.  The  whole 
plant,  great  as  is  its  size,  and  made  up  though 
it  seems  to  be  of  many  groups  of  mor- 
phological units,  united  into  a  compound 
group  by  their  marked  subordination  to  a 
connecting  mass,  is  nevertheless  a  single 
thallus.  The  aggregate  is  still  an  aggregate 
of  the  second  order. 
But  amco.g  certain  of  the  highest  Algce,  we  do  find  some- 
thing more  than  this  union  of  the  pseud-axial  with  the 
pseudo-foliar  structure.  In  addition  to  pseud-axes  of 
comparative  complexity ;  and  in  addition  to  pseudo-folia 
that  are  like  leaves,  not  only  in  their  general  shapes,  but 
in  having  mid-ribs  and  even  veins ;  there  are  the  be- 
ginnings of  a  higher  stage  of  integration.  Figs.  38,  39, 
and  40,  show  some  of  the  steps.  In  Rhodymenia  palmata, 
Fig.  38,  the  parent-frond  is  comparatively  irregular  in  shape, 
and  without  a  mid-rib  ;  and  along  with  this  very  imperfect 
integration,  we  see  that  the  secondary  fronds  growing  from 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS. 


23 


the  edges  are  distributed  very  much  at  random,  and  are  by 
no  means  specific  in  their  shapes.  A  considerable  advance  is 
displayed  by  Phyttophora  rubens,  Fig.  39.  Here  the  frond, 
primary,  secondary,  or  tertiary,  betrays  some  approach  to- 
wards regularity  in  both  form  and  size ;  by  which,  as  also  by 
its  partially-developed  mid-rib,  there  is  established  a  more 
marked  individuality ;  and  at  the  same  time,  the  growth  of 
the  secondary  fronds  no  longer  occurs  anywhere  on  the  edge, 
in  the  same  plane  as  the  parent  frond,  but  from  the  surface 
at  specific  places.  Delesseria  sanguined,  Fig.  40,  illustrates 
a  much  more  definite  arrangement  of  the  same  kind.  The 
fronds  of  this  plant,  quite  regularly  shaped,  have  their  parts 
decidedly  subordinated  to  the  whole  ;  and  from  their  mid- 
ribs grow  other  fronds,  which  are  just  like  them.  Each  of 
these  fronds  is  an  organized  group  of  those  morphological 
units  which  we  distinguish  as  aggregates  of  the  first  order. 
And  in  this  case,  two  or  more  such  aggregates  of  the  second 
order,  well  individuated  by  their  forms  and  structures,  are 
anitcd  together ;  and  the  plant  composed  of  them  is  thus 
rendered,  in  so  far,  an  aggregate  of  the  third  order. 

Just  noting  that  in  certain  of  the  most-developed  Alga,  as 


21  MOKPHOLOGICAL    DEVELOPMENT. 

the  Sargassum,  or  common  gulf- weed,  this  tertiary  degree  of 
composition  is  far  more  completely  displayed,  so  as  to  pro- 
duce among  Thallogens  a  type  of  structure  closely  simulating 
that  of  the  higher  plants,  let  us  now  pass  to  the  considera- 
tion of  these  higher  plants. 

§  186.  Having  the  surface  of  the  soil  for  a  support  and  tho 
air  for  a  medium,  terrestrial  plants  are  mechanically  circum- 
stanced in  a  manner  widely  different  from  that  in  which 
aquatic  plants  are  circumstanced.  Instead  of  being  buoyed 
up  by  a  surrounding  fluid  of  specific  gravity  equal  to  their 
own,  they  have  to  erect  themselves  into  a  rare  fluid  which 
yields  no  appreciable  support.  Further,  they  are  dis- 
similarly conditioned  in  having  two  sources  of  nutriment  in 
place  of  one.  Unlike  the  Alga,  which  derive  all  the  mate- 
rials for  their  tissues  from  the  water  bathing  their  entire 
surfaces,  and  use  their  roots  only  for  attachment ;  most  of  the 
plants  which  cover  the  Earth's  surface,  absorb  part  of  their 
food  through  their  imbedded  roots  and  part  through  their 
exposed  leaves.  These  two  marked  unlikenesses  in  the  rela- 
tions to  surrounding  conditions,  profoundly  affect  the  respec- 
tive modes  of  growth.  "We  must  duly  bear  them  in  mind 
while  studying  the  further  advance  of  composition. 

The  class  of  plants  to  which  we  now  turn — that  of  Acrogens 
— is  nearly  related  by  its  lower  members  to  the  classes  above 
dealt  with  :  so  much  so,  that  some  of  the  inferior  liverworts 
are  quite  licheniform,  and  are  often  mistaken  for  lichens. 
Passing  over  these,  let  us  recommence  our  analysis  with  such 
members  of  the  class,  as  repeat  those  indications  of  progress 
towards  a  higher  composition,  which  we  have  just  observed 
among  the  more-developed  Algce.  The  Jungermanniacece 
furnish  us  with  a  series  of  types,  clearly  indicating  the  transi- 
tion from  an  aggregate  of  the  second  order  to  an  aggregate 
of  the  third  order.  Figs.  41,  and  42,  indicate  the  structure 
among  the  lowest  of  this  group.  Here  there  is  but  an  incom- 
plete development  of  the  second  order  of  aggregate.  The 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  25 

frond  grows  as  irregularly  as  the  thallus  of  a  lichen :  it  is  in- 
definite in  size  and  outline,  spreading  hither  or  thither  as  the 
conditions  favour.  Moreover,  it  lacks  the  differentiations  re- 


quired to  subordinate  its  parts  to  the  whole :  it  is  uniformly 
cellular,  having  neither  mid-rib  nor  veins  ;  and  it  puts  out 
rootlets  indifferently  from  all  parts  of  its  under-surface.  In 
Fig  43,  Jungermannia  epipkylla,  we  have  an  advance  on  this 
type.  There  is  here,  as  shown  in  the  transverse  section,  Fig. 
44,  a  thickening  of  the  frond  along  its  central  portion,  pro- 
ducing something  like  an  approach  towards  a  mid- rib ;  and 
from  this  the  rootlets  are  chiefly  given  off.  The  outline,  too, 
is  much  less  irregular ;  whence  results  greater  distinctness 
of  the  individuality.  A  further  step  is  displayed  in  Junger- 
mannia fur  cata,  Fig.  45.  The  frond  of  this  plant,  compara- 
tively well  integrated  by  the  distribution  of  its  substance 
around  a  decided  mid-rib,  and  by  its  comparatively-definite 
outlines,  produces  secondary  fronds — there  is  what  is  called 
proliferous  growth  ;  and,  occasionally,  as  shown  in  Fig.  46, 
representing  an  enlarged  portion,  the  growth  is  doubly-pro- 
liferous. In  these  cases,  however,  the  tertiary  aggregate,  so 
far  as  it  is  formed,  is  but  very  feebly  integrated ;  and  its  in- 
tegration is  but  temporary.  For  not  only  do  these  younger 
fronds  that  bud  out  from  the  mid-ribs  of  older  fronds,  develop 
rootlets  of  their  own  ;  but  as  soon  as  they  are  well  grown  and 
adequately  rooted,  they  dissolve  their  connexions  with  the 
parent- fronds,  and  become  quite  independent.  From 

these  transitional  forms  we  pass,  in  the  higher  Jungerman- 
niacea?,  to  forms  composed  of  many  fronds  that  are  perman- 
ently united  by  a  continuous  stem.  A  more-developed  ag- 


26  MORPHOLOGICAL   DEVELOPMENT. 


gregate  of  the  third  order  is  thus  produced.  But  though, 
along  with  increased  definiteness  in  the  secondary  aggregates, 
there  is  here  an  integration  of  them  so  extensive  and  so  re- 
gular, that  they  are  visibly  subordinated  to  the  whole  they 
form ;  yet  the  subordination  is  really  very  incomplete.  In 
some  instances,  as  in  /.  complanata,  Fig.  47,  the  leaflets  de- 
velop roots  from  their  under  surfaces,  just  as  the  primitive 
frond  does;  and  in  the  majority  of  the  group,  as  in  J. 
capitata,  Fig.  48,  roots  are  given  off  all  along  the  connecting 
stem,  at  the  spots  where  the  leaflets  or  frondlets  join  it :  the 
result  being,  that  though  the  connected  frondlets  form  a 
physical  whole,  they  do  not  form,  in  any  decided  manner, 
a  physiological  whole;  since  successive  portions  of  the 
united  series,  carry  on  their  functions  independently  of  the 
rest.  Finally,  the  most  developed  members  of  the 

group,  present  us  with  tertiary  aggregates  that  are  physio- 
logically as  well  as  physically  integrated.  Not  lying  prone 
like  the  kinds  thus  far  described,  but  growing  erect,  the  stem 
and  attached  leaflets  become  dependent  upon  a  single  root  or 
group  of  roots ;  and  being  so  prevented  from  carrying  on  their 
functions  separately,  are  made  members  of  a  compound  indi- 
vidual— there  arises  a  definitely-established  aggregate  of  the 
third  degree  of  composition. 

The  facts  as  arranged  in  the  above  order,  are  suggestive. 
Minute  aggregates,  or  cells,  the  grouping  of  which  we  traced 
in  §  182,  showed  us  analogous  phases  of  indefinite  union, 
which  appeared  to  lead  the  way  towards  definite  union.  "We 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  21 

see  here  among  compound  aggregates,  as  we  saw  there 
among  simple  aggregates,  the  establishment  of  a  specific 
form,  and  a  size  that  falls  within  moderate  limits  of  varia- 
tion. This  passage  from  less  definite  extension  to  more  de- 
finite extension,  seems  in  the  one  case,  as  the  other,  to  be  ac- 
companied by  the  result,  that  growth  exceeding  a  certain 
rate,  ends  in  the  formation  of  a  new  aggregate,  rather  than 
an  enlargement  of  the  old.  And  on  the  higher  stage,  as  oa 
the  lower,  this  process,  irregularly  carried  out  in  the  simpler 
types,  produces  in  them  unions  that  are  but  temporary  ;  while 
in  the  more- developed  types,  it  proceeds  in  a  systematic  way, 
and  ends  in  the  production  of  a  permanent  aggregate  that  is 
doubly  compound. 

Must  we  then  conclude,  that  as  cells,  or  morphological 
units,  are  integrated  into  a  unit  of  a  higher  order,  which  we 
call  a  thallus  or  frond ;  so,  by  the  integration  of  fronds,  there 
is  evolved  a  structure  such  as  the  above-delineated  species 
possess  ?  "Whether  this  is  the  interpretation  to  be  given  of 
these  plants,  we  shall  best  see  when  considering  whether  it  is 
the  interpretation  to  be  given  of  plants  that  rank  above  them. 
Thus  far  we  have  dealt  only  with  the  Cryptogamia.  TVe 
have  now  to  deal  with  the  Phanerogamia  or  PhaDnogamia. 


CHAPTER  HI. 

THE   MORPHOLOGICAL   COMPOSITION   OF   PLAJST8, 
CONTINUED. 

§  187.  THAT  advanced  composition  arrived  at  in  the 
Acrogens,  is  carried  still  further  in  the  Endogens  and  Exo- 
gens.  In  these  most- elevated  vegetal  forms,  aggregation 
of  the  third  order  is  always  distinctly  displayed ;  and  aggre- 
gates of  the  fourth,  fifth,  sixth,  &c.,  orders  are  very  common. 

Our  inquiry  into  the  morphology  of  these  flowering 
plants,  may  be  advantageously  commenced  by  studying  the 
development  of  simple  leaves  into  compound  leaves.  It  is 
easy  to  trace  the  transition,  as  well  as  the  conditions  under 
which  it  occurs ;  and  tracing  it  will  prepare  us  for  under- 
standing how,  and  when,  metamorphoses  still  greater  in  de- 
gree, take  place. 

§  188.  If  we  examine  a  branch  of  the  common  bramble, 
when  in  flower  or  afterwards,  we  shall  not  unfrequently  find 
a  simple  or  undivided  leaf,  at  the  insertion  of  one  of  the 
lateral  flower-bearing  axes,  composing  the  terminal  cluster  of 
flowers.  Sometimes  this  leaf  is  partially  lobed ;  sometimes 
cleft  into  three  small  leaflets.  Lower  down  on  the  shoot,  if 
it  be  a  lateral  one,  occur  larger  leaves;  composed  of  three 
leaflets ;  and  in  some  of  these,  two  of  the  leaflets  may  be 
lobed  more  or  less  deeply.  On  the  main  stem,  the  leaves, 
usually  still  larger,  will  be  found  to  have  five  leaflets.  Sup- 


THE    MORPHOLOGICAL   COMPOSITION    OF    PLANTS. 


29 


posing  the  plant  to  be  a  well-grown  one,  it  will  furnish  all 
gradations  between  the  simple,  very  small  leaf,  and  the  large 
composite  leaf,  containing  sometimes  even  seven  leaflets. 
Figs.  50  to  64,  represent  leading  stages  of  the  transition. 


What  determines  this  transition  ?  Observation  shows  that  the 
quintuple  leaves  occur  where  the  materials  for  growth  are 
supplied  in  greatest  abundance ;  that  the  leaves  become  less 


30  MORPHOLOGICAL    DEVELOPMENT. 

and  less  compound,  in  proportion  to  their  remoteness  from  the 
main  currents  of  sap ;  and  that  where  an  entire  absence  of 
divisions   or  lobes  is  observed,  it  is  on  leaves  within  the 
flower-bunch :    at  the  place,  that  is,  where  the  forces  that 
cause  growth   are    nearly  equilibrated   by  the  forces  that 
oppose  growth;  and  where,  as  a  consequence,  gamogenesis  is 
about  to  set  in  (§  78).     Additional  evidence  that  the  degree 
of  nutrition  determines  the  degree  of  composition  of  the  leaf, 
is  furnished  by  the  relative  sizes  of  the  leaves.     Not  only,  on 
the  average,  is  the  quintuple  leaf  much  larger  in  its  total  area 
than  the  triple  leaf;  but  the  component  leaflets  of  the  one,  are 
usually  much  larger  than  those  of  the  other.     The  like  con- 
trasts are  still  more  marked  between  triple  leaves  and  simple 
leaves.     This  connexion  of  decreasing  size  with  decreasing 
composition,  is  conspicuous  in  the  series  of  figures :  the  differ- 
ences shown,  being  not  nearly  so  great  as  may  bo  frequently 
observed.     Confirmation  may  be  drawn  from  the  fact,  that 
when  the  leading  shoot  is  broken  or  arrested  in  its  growth, 
the  shoots  it  gives  off  (provided  they  are  given  off  after  the 
injury),  and  into  which  its  checked  currents  of  sap  are  thrown, 
produce  leaves  of  five  leaflets,  where  ordinarily  leaves  of  three 
leaflets  occur.     Of  course  incidental  circumstances,  as  varia- 
tions in  the  amounts  of  sunshine,  or  of  rain,  or  of  matter  sup- 
plied to  the  roots,  are  ever  producing  changes  in  the  state  of 
the  plant  as  a  whole ;  and  by  thus  affecting  the  nutrition  of  its 
leaf-buds  at  the  times  of  their  formation,  cause  irregularities 
in  the  relations  of  size  and  composition  above  described.    But 
taking  these  causes  into  account,  it  is  abundantly  manifest 
that  a  leaf-bud  of  the  bramble,  will  develop  into  a  simple 
leaf  or  into  a  leaf  compounded  in  different  degrees,  according 
to  the  quantity  of  assimilable  matter  brought  to  it  at  the 
time  when  the  rudiments  of  its    structure  are  being  fixed. 
And  on  studying  the  habits  of  other  plants — on  observing 
how  annuals  that  have  compound  leaves,  usually  bear  simple 
leaves  at  the  outset,  when  the  assimilating  surface  is  but 
small ;  and  how,  when  compound-leaved  plants  in  full  growth 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  31 

oear  simple  leaves  in  the  midst  of  compound  ones,  the  rela- 
tive smallness  of  such  simple  leaves  shows  that  the  buds  from 
which  they  arose  were  ill-supplied  with  sap ;  it  will  cease  to 
be  doubted  that  a  foliar  organ  may  be  metamorphosed  into  a 
group  of  foliar  organs,  if  furnished,  at  the  right  time,  with 
a  quantity  of  matter  greater  than  can  be  readily  organized 
round  a  single  centre  of  growth.  An  examination  of  the 
transitions  through  which  a  compoxmd  leaf  passes  into  a 
doubly-compound  leaf,  as  seen  in  the  various  intermediate 
forms  of  leaflets  in  Fig.  65,  will  further  enforce  this 
conclusion. 


65 


Here  we  may  advantageously  note,  too,  how  in  such  cases, 
the  leaf- stalk  undergoes  concomitant  changes  of  structure. 
In  the  bramble-leaves  above  described,  it  becomes  compound 
simultaneously  with  the  leaf — the  veins  become  mid-ribs  while 
the  mid-ribs  become  petioles.  Moreover,  the  secondary  stalks, 
and  still  more  the  main  stalks,  bear  thorns  similar  in  their 
shapes,  and  approaching  in  their  sizes,  to  those  on  the  stem; 


32  MORPHOLOGICAL   DEVELOPMENT. 

besides  simulating  the  stem  in  colour  and  texture.  In  the 
petioles  of  large  compound  leaves,  like  those  of  the  com- 
mon Heraclcum,  we  still  more  distinctly  see  both  internal 
and  external  approximations  in  character  to  axes.  Nor  are 
there  wanting  plants  whose  large,  though  simple,  leaves,  are 
held  out  far  from  the  stems,  by  foot-stalks  that  are,  near  the 
ends,  sometimes  so  like  axes,  that  the  transverse  sections  of 
the  two  are  indistinguishable ;  as  instance  the  Calla  Etliiopica. 
One  other  fact  respecting  the  modifications  which  leaves 
undergo,  should  be  set  down.  Not  only  may  leaf- stalks  as- 
sume to  a  great  degree  the  characters  of  steins,  when  they 
have  to  discharge  the  functions  of  stems,  by  supporting  many 
leaves  or  very  large  leaves ;  but  they  may  assume  the  cha- 
racters of  leaves,  when  they  have  to  undertake  the  functions 
of  leaves.  The  Australian  Acacias  furnish  a  remarkable 
illustration  of  this.  Acacias  elsewhere  found,  bear  pinnate 
leaves ;  but  the  majority  of  those  found  in  Australia,  bear  what 
appear  to  be  simple  leaves.  It  turns  out,  however,  that  these 
are  merely  leaf-stalks  flattened  out  into  foliar  shapes :  the 
laminae  of  the  leaves  being  undeveloped.  And  the  proof 
is,  that  in  young  plants,  showing  their  kinships  by  their  em- 
bryonic characters,  these  leaf- like  petioles  bear  true  leaflets  at 
their  ends.  A  metamorphosis  of  like  kind  occurs  in  Oxalis 
bupleurifolia,  Fig.  66.  The  fact  most  deserving  of  notice, 
however,  is,  that  these  leaf- 
66  ($L-—  sta^s'  ™  usurping  the  gene- 

l  aspects  and  functions  of 
leaves,  have  also  usurped  their 
structures :  though  their  ve- 
nation is  not  like  that  of  the  leaves  they  replace,  yet  they 
have  veins,  and  in  some  cases  mid-ribs. 

Reduced  to  their  most  general  expression,  the  truths 
above  shadowed  forth  are  these : — That  group  of  morphologi- 
cal units,  or  cells,  which  we  see  integrated  into  the  compound 
unit  called  a  leaf,  has,  in  each  higher  plant,  a  typical  form;  due 
to  the  special  arrangement  of  these  cells  around  a  mid-rib  and 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  33 

veins.  If  the  multiplication  of  morphological  units,  at  the 
time  when  the  leaf- bud  is  taking  on  its  main  outlines,  exceeds 
a  certain  limit,  these  units  begin  to  arrange  themselves  round 
secondary  centres,  or  lines  of  growth,  in  such  ways  as  to  re- 
peat, in  part  or  wholly,  the  typical  form :  the  larger  veins 
become  transformed  into  imperfect  mid- ribs  of  partially  inde- 
pendent leaves ;  or  into  complete  mid-ribs  of  quite  separate 
leaves.  And  as  there  goes  on  this  transition  from  a  single 
aggregate  of  cells  to  a  group  of  such  aggregates,  there  simul- 
taneously arises,  by  similarly-insensible  steps,  a  distinct 
structure  which  supports  the  several  aggregates  thus  pro- 
duced, and  unites  them  into  a  compound  aggregate.  These 
phenomena  should  be  carefully  studied ;  since  they  give  us  a 
key  to  more  involved  phenomena. 

§  189.  Thus  far  we  have  dealt  with  leaves  ordinarily  so 
called :  briefly  indicating  the  homologies  between  the  parts  of 
the  simple  and  the  compound.  Let  us  now  turn  to  the  homo- 
logies among  foliar  organs  in  general.  These  have  been 
made  familiar  to  readers  of  natural  history,  by  popularized 
outlines  of  "  The  Metamorphosis  of  Plants  " — a  title,  by  the 
way,  which  is  far  too  extensive ;  since  the  phenomena  treated 
of  under  it,  form  but  a  small  portion  of  those  it  properly  in- 
cludes. 

Passing  over  certain  vague  anticipations  that  have  been 
quoted  from  ancient  writers,  and  noting  only  that  some 
clearer  recognitions  were  reached  by  Joachim  Jung,  a  Ham- 
burg professor,  ^n  the  middle  of  the  17th  century  ;  we  come 
to  the  Theoria  Generation-is,  which  Wolff  published  in  1759, 
and  in  which  he  gives  a  definite  form  to  the  conceptions  that 
have  since  become  current.  Specifying  the  views  of  "Wolff, 
Dr  Masters  writes, — "  After  speaking  of  the  homologous 
nature  of  the  leaves,  the  sepals  and  petals,  an  homology 
consequent  on  their  similarity  of  structure  and  identitv  of 
origin,  he  goes  on  to  state  that  the  '  pericarp  is  manifestly 
composed  of  several  leaves,,  as  in  the  calyx,  with  this  differ- 


34  MORPHOLOGICAL   DEVELOPMENT. 

ence  only,  that  the  leaves  which  are  merely  placed  in  close 
contact  in  the  calyx,  are  here  united  together ; '  a  view  which 
he  corroborates  by  referring  to  the  manner  in  which  many 
capsules  open  and  separate  '  into  their  leaves.'  The  seeds,  too, 
he  looks  upon  as  consisting  of  leaves  in  close  combination.  His 
reasons  for  considering  the  petals  and  stamens  as  homologous 
with  leaves,  are  based  upon  the  same  facts  as  those  which  led 
Linnaeus,  and,  many  years  afterwards,  Goethe,  to  the  same 
conclusion.  '  In  a  word/  says  Wolff,  '  we  see  nothing  in 
the  whole  plant,  whose  parts  at  first  sight  differ  so  remark- 
ably from  each  other,  but  leaves  and  stem,  to  which  latter 
the  root  is  referrible.'  "  It  appears  that  Wolff,  too,  enunci- 
ated the  now-accepted  interpretation  of  compound  fruits  : 
basing  it  on  the  same  evidence  as  that  since  assigned.  In 
the  essay  of  Goethe,  published  thirty  years  after,  these  rela- 
tions among  the  parts  of  flowering  plants  were  traced  out  in 
greater  detail,  but  not  in  so  radical  a  way ;  for  Goethe  did 
not,  as  did  Wolff,  verify  his  hypothesis  by  dissecting  buds  in 
their  early  stages  of  development.  Goethe  appears  to  have 
arrived  at  his  conclusions  independently.  But  that  they  were 
original  with  him,  and  that  he  gave  a  more  variously-illus- 
trated exposition  of  them  than  had  been  given  by  Wolff, 
does  not  entitle  him  to  anything  beyond  a  secondary  place, 
among  those  who  have  established  this  important  generaliza- 
tion. 

Were  it  not  that  these  pages  may  be  read  by  some  to 
whom  Biology,  in  all  its  divisions,  is  a  new  subject  of  study,  it 
would  be  needless  to  name  the  evidence  on  which  this  now- 
familiar  generalization  rests.  For  the  information  of  such 
it  will  suffice  to  say,  that  the  fundamental  kinship  existing 
among  all  the  foliar  organs  of  a  flowering  plant,  is  shown  by 
the  transitional  forms  which  may  be  traced  between  them, 
and  by  the  occasional  assumption  of  one  another's  forms. 
"  Floral  leaves,  or  bracts,  are  frequently  only  to  be  distin- 
guished from  ordinary  leaves  by  their  position  at  the  base  of 
the  flower ;  at  ether  times  the  bracts  gradually  assume  more 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  35 

and  more  of  the  appearance  of  the  sepals."  The  sepals,  or 
divisions  of  the  calyx,  are  not  unlike  undeveloped  leaves: 
sometimes  assuming  quite  the  structures  of  leaves.  In  other 
cases,  they  acquire  partially  or  wholly  the  colours  of  the 
petals — as,  indeed,  the  bracts  and  uppermost  stem-leaves 
occasionally  do.  Similarly,  the  petals  show  their  alliances  to 
the  foliar  organs  lower  down  on  the  axis,  and  to  those  higher 
up  on  the  axis  :  on  the  one  hand,  they  may  develop  into  or- 
dinary leaves  that  are  green  and  veined ;  and,  on  the  other 
hand,  as  so  commonly  seen  in  double  flowers,  they  may  bear 
anthers  on  their  edges.  All  varieties  of  gradation  into 
neighbouring  foliar  organs,  may  be  witnessed  in  stamsns. 
Flattened  and  tinted  in  various  degrees,  they  pass  insensibly 
into  petals,  and  through  them  prove  their  homology  with 
leaves ;  into  which,  indeed,  they  are  transformed  in  flowers 
that  become  wholly  foliaceous.  The  style,  too,  is  occasionally 
changed  into  petals  or  into  green  leaflets ;  and  even  the 
ovules  are  now  and  then  seen  to  take  on  leaf-like  forms. 
Thus  we  have  clear  evidence  that  in  Phaenogams,  all  the  ap- 
pendages of  the  axis  are  homologues  :  they  are  all  modified 
leaves. 

AYolff  established,  and  Goethe  further  illustrated,  another 
general  law  of  structure  in  flowering  plants.  Each  leaf 
commonly  contains  in  its  axil,  a  bud,  similar  in  structure  to 
the  terminal  bud  This  axillary  bud  may  remain  unde- 
veloped; or  it  may  develop  into  a  lateral  shoot  like  the 
main  shoot ;  or  it  may  develop  into  a  flower.  If  a  shoot 
bearing  lateral  flowers  be  examined,  it  will  be  found  that  the 
internode,  or  space  which  separates  each  leaf  with  its  axillary 
flower  from  the  leaf  and  axillary  flower  above  it,  becomes 
gradually  less  towards  the  upper  end  of  the  shoot.  In  some 
plants,  as  in  the  fox-glove,  the  internodes  constitute  a 
regularly-diminishing  series.  In  other  plants,  the  series  they 
form  suddenly  begins  to  diminish  so  rapidly,  as  to  bring  the 
flowers  into  a  short  spike — instance  the  common  orchis.  And 
again,  by  a  still  more  sudden  dwarfing  of  the  internodes,  the 


36  MORPHOLOGICAL    DEVELOPMENT. 

flowers  are  brought  into  a  cluster  ;  as  they  are  in  the  cows- 
lip. On  contemplating  a  clover-flower,  in  which  this 
clustering  has  been  carried  so  far  as  to  produce  a  com- 
pact head ;  and  on  considering  what  must  happen  if,  by  a 
further  arrest  of  axial  development,  the  foot-stalks  of  the 
florets  disappear ;  it  will  be  seen  that  there  must  result  a 
crowd  of  flowers,  seated  close  together  on  the  end  of  the  axis. 
And  if,  at  the  same  time,  the  internodes  of  the  upper  stem- 
leaves  also  remain  undeveloped,  these  stem-leaves  will  be 
grouped  into  a  common  calyx  or  involucre :  we  shall  have  a 
composite  flower,  such  as  the  thistle.  Hence,  to  modifications 
in  the  developments  of  foliar  organs,  have  to  be  added  modi- 
fications in  the  developments  of  axial  organs.  Comparisons 
disclose  the  gradations  through  which  axes,  like  their  append- 
ages, pass  into  all  varieties  of  size,  proportion,  and  structure. 
And  we  learn  that  the  occurrence  of  these  two  kinds  of 
metamorphosis,  in  all  conceivable  degrees  and  combinations, 
furnishes  us  with  a  proximate  interpretation  of  morpho- 
logical composition  in  Phtenogams. 

I  say  a  proximate  interpretation,  because  there  remain 
to  be  solved  certain  deeper  problems  ;  one  of  which  at  once 
presents  itself  to  be  dealt  with  under  the  present  head. 
Leaves,  petals,  stamens,  &c.,  being  shown  to  be  homologous 
foliar  organs ;  and  the  part  to  which  they  are  attached, 
proving  to  be  an  indefinitely- extended  axis  of  growth,  or 
axial  organ ;  we  are  met  by  the  questions, — What  is  a  foliar 
organ  ?  and  What  is  an  axial  organ  ?  The  morphological  com- 
position of  a  Phaenogam  is  undetermined,  so  long  as  we  can- 
not say  to  what  lower  structures  leaves  and  shoots  are  homo- 
logous ;  and  how  this  integration  of  them  originates.  To 
these  questions  let  us  now  address  ourselves. 

§  190-1.  Already,  in  §  78,  reference  has  been  made  to  the 
occasional  development  of  foliar  organs  into  axial  organs: 
the  special  case  there  described,  being  that  of  a  fox-glove,  in 
which  some  of  the  sepals  were  replaced  by  flower-buds. 


THE    MORPHOLOGICAL   COMPOSITION    OF    PLANTS.     37 4:3 

The  observation  of  these  and  some  analogous  monstrosities, 
raising  the  suspicion  that  the  distinction  between  foliar 
organs  and  axial  organs  is  not  absolute,  led  me  to  examine 
into  the  matter ;  and  the  result  has  been  the  deepening  of 
this  suspicion  into  a  conviction.  Part  of  the  evidence  is  given 
in  Appendix  A 

Some  time  after  having  reached  this  conviction,  I  found  on 
looking  into  the  literature  of  the  subject,  that  analogous  ir- 
regularities have  suggested  to  other  observers,  beliefs  similarly 
at  variance  with  the  current  morphological  creed.  Diffi- 
culties in  satisfactorily  denning  these  two  elements,  have 
served  to  shake  this  creed  in  some  minds.  To  others, 
the  strange  leaf-like  developments  which  axes  undergo  in 
certain  plants,  have  afforded  reasons  for  doubting  the 
constancy  of  this  distinction  Avhich  vegetal  morphologists 
usually  draw.  And  those  not  otherwise  rendered  sceptical, 
have  been  made  to  hesitate  by  such  cases  as  that  of  the 
Nepaul-barley ;  in  which  the  glume,  a  foliar  organ,  becomes 
developed  into  an  axis,  and  bears  flowers.  In  his  essay — 
"  Vegetable  Morphology  :  its  History  and  Present  Condi- 
tion," *  whence  I  have  already  quoted,  Dr  Masters  indicates 
sundry  of  the  grounds  for  thinking,  that  there  is  no  impassable 
demarcation  between  leaf  and  stem.  Among  other  difficult- 
ies which  meet  us  if  we  assume  that  the  distinction  is  abso- 
lute, he  asks — "  What  shall  we  say  to  cases  such  as  those 
afforded  by  the  leaves  of  Guarea  and  Trichilia,  where  the 
leaves  after  a  time  assume  the  condition  of  branches  and  de- 
velop young  leaflets  from  their  free  extremities,  a  process  less 
perfectly  seen  in  some  of  the  pinnate-leaved  kinds  of  Berber!* 
or  Mahonia,  to  be  found  in  almost  every  shrubbery  ?  " 

A  class  of  facts  on  which  it  will  be  desirable  for  us  nere  to 
dwell  a  moment,  before  proceeding  to  deal  with  the  matter 
deductively,  is  presented  by  the  Cadacece.  In  this  remark- 
able group  of  plants,  deviating  in  such  varied  ways  from  the 
ordinary  phaonogamic  type,  we  find  many  highly  instructive 

*  See  firilith  and  Foreign  Medico-Chirwgical  Review  for  January,  1862. 


44  MORPHOLOGICAL    DEVELOPMENT. 

modifications  of  form  and  structure.  By  contemplating  the 
changes  here  displayed  within  the  limits  of  a  single  order, 
we  shall  greatly  widen  our  conception  of  the  possibilities  of 
metamorphosis  in  the  vegetal  kingdom,  taken  as  a  whole. 
Two  different,  but  similarly-significant,  truths  are  illustrated. 
First,  we  are  shown  how,  of  these  two  components  of  a 
flowering  plant,  commonly  regarded  as  primordially  distin- 
guished, one  may  assume,  throughout  numerous  species,  the 
functions,  and  to  a  great  degree  the  appearance,  of  the  other. 
Second,  we  are  shown  how,  in  the  same  individual,  there 
may  occur  a  re-metamorphosis — the  usurped  function  and 
appearance  being  maintained  in  one  part  of  the  plant,  while 
in  another  part,  there  is  a  return  to  the  ordinary  appearance 
and  function.  We  will  consider  these  two  truths  separ- 
ately. Some  of  the  Eiipkorliacea,  which  simulate 
Cactuses,  show  us  the  stages  through  which  such  abnormal 
structures  are  arrived  at.  In  EupJiorbia  splendens,  the  lateral 
axes  are  considerably  swollen  at  their  distal  ends,  so  as  often 
to  be  club-shaped :  still,  however,  being  covered  with  bark 
of  the  ordinary  colour,  and  still  bearing  leaves.  But 
in  kindred  plants,  as  Euphorbia  neriifolia,  this  swelling  of 
the  lateral  axes  is  carried  to  a  far  greater  extent ;  and,  at 
the  same  time,  a  green  colour  and  a  fleshy  consistence  have 
been  acquired :  the  typical  relations  nevertheless  being  still 
shown,  by  the  few  leaves  that  grow  out  of  these  soft  and 
swollen  axes.  In  the  Cactacece,  which  are  thus  resembled  by 
plants  not  otherwise  allied  to  them,  we  have  indications  of  a 
parallel  transformation.  Some  kinds,  not  commonly  brought 
to  England,  bear  leaves ;  but  in  the  species  most  familiar  to 
us,  the  leaves  are  undeveloped  and  the  axes  assume  their 
functions.  Passing  over  the  many  varieties  of  form  and 
combination  which  these  green  succulent  growths  display,  we 
have  to  note  that  in  some  genera,  as  in  Phyllocadns,  they 
become  flattened  out  into  foliaceous  shapes,  having  mid-ribs 
and  something  approaching  to  veins.  So  that  here,  and  in 
the  genus  Epiphyllum,  which  has  this  character  still  more 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  45 

marked,  the  plant  appears  to  be  composed  of  fleshy 
Leaves  growing  one  upon  another.  And  then,  in  llhipsalis, 
the  same  parts  are  so  leaf-like  that  an  uncritical  observer 
would  regard  them  as  leaves.  These  which  are  axial  organs 
in  their  homologies,  have  become  foliar  organs  in  their 
analogies.  When,  instead  of  comparing  these 

strangely-modified  axes  in  different  genera  of  Cactuses,  we 
compare  them  in  the  same  individual,  we  meet  with  transform- 
ations no  less  striking.  Where  a  tree-like  form  is  pro- 
duced by  the  growth  of  these  foliaceous  shoots,  one  on  another ; 
and  where,  as  a  consequence,  the  first-formed  of  them  become 
the  main  stem  that  acts  as  support  to  secondary  and  tertiary 
stems ;  they  lose  their  green,  succulent  character,  acquire 
bark,  and  become  woody — in  resuming  the  functions  of  axes 
they  resume  the  structures  of  axes,  from  which  they  had  de- 
viated. In  Fig.  71  are  shown  some  of  the  leaf-like  axes  of 
Rliipsalis  rhombea  in  their  young  state ;  while  Fig.  72  repre- 
sents the  oldest  portion  of  the 
same  plant,  in  which  the  foli- 
aceous characters  are  quite 
obliterated,  and  there  has  re- 
sulted an  ordinary  stem-struc- 
ture. One  further  < 
fact  is  to  be  noted.  At  the* 
same  time  that  their  leaf-like  appearances  are  lost,  the 
axes  also  lose  their  separate  individualities.  As  they  become 
stem-like,  they  also  become  integrated ;  and  they  do  this  so 
effectually,  that  their  original  points  of  junction,  at  first  so 
strongly  marked,  are  effaced,  and  a  consolidated  trunk  is 
produced. 

Joined  with  the  facts  previously  specified,  these  facts 
help  us  to  conceive  how,  in  the  evolution  of  flowering  plants 
in  general,  the  morphological  components  that  were  once 
distinct,  may  become  extremely  disguised.  We  may  ration- 
ally expect  that  during  so  long  a  course  of  modification, 
much  greater  changes  of  form,  and  much  more  decided  fusions 


46  MORPHOLOGICAL    DEVELOPMENT. 

of  parts,  have  taken  place.  Seeing  how,  in  an  individual 
plant,  the  single  leaves  pass  into  compound  leaves,  by  the  devel- 
opment of  their  veins  into  mid-ribs  while  their  mid-ribs  begin 
to  simulate  axes ;  and  seeing  that  leaves  ordinarily  exhibit- 
ing definitely-limited  developments,  occasionally  produce 
other  leaves  from  their  edges ;  we  are  led  to  suspect  the  pos- 
sibility of  still  greater  changes  in  foliar  organs.  When,  fur- 
ther, we  find  that  within  the  limits  of  one  natural  order, 
petioles  usurp  the  functions  and  appearances  of  leaves,  at  the 
same  time  that  in  other  orders,  as  in  Ruscus,  lateral  axes  so 
completely  simulate  leaves  that  their  axial  nature  would  never 
have  been  supposed,  did  they  not  bear  flowers  on  their  mid- 
ribs or  edges ;  and  when,  among  Cactuses,  we  perceive  that 
such  metamorphoses  and  re-metamorphoses  take  place  with 
great  facility ;  our  suspicion  that  the  morphological  elements 
of  Phaenogams  admit  of  profound  transformations,  is 
deepened.  And  then,  on  discovering  how  frequent  are  the 
monstrosities  that  do  not  seem  satisfactorily  explicable  without 
admitting  the  development  of  foliar  organs  into  axial  organs ; 
we  become  ready  to  entertain  the  hypothesis,  that  during  the 
p. volution  of  the  phaenogamic  type,  the  distinction  between 
leaves  and  axes  has  arisen  by  degrees. 

With  our  pre-conceptions  loosened  by  such  facts,  and 
carrying  with  us  the  general  idea  which  such  facts  suggest, 
let  us  now  consider  in  what  way  the  typical  structure  of  a 
flowering  plant  may  be  interpreted. 

§  192.  To  proceed  methodically,  we  must  seek  a  clue  to 
the  structures  of  Endogens  and  Exogens,  in  the  structures 
of  those  inferior  plants  that  approach  to  them — Acrogens. 
The  various  divisions  of  this  class  present,  along  with  sundry 
characters  which  ally  them  with  Thallogens,  other  charac- 
ters by  which  the  phaenogamic  structure  is  shadowed  forth. 
While  some  of  the  inferior  Hepaticce  or  Liverworts,  severally 
consist  of  little  more  than  a  thallus-like  frond ;  among  the 
higher  members  of  this  group,  and  still  more  among  the 


lira   MOHrilOLOGICAL   COMPOSITION   OF    PLANTS.  47 

Mosses  and  Ferns,  we  find  a  distinctly  marked  stem.*  Some 
Acrogeiis  have  foliar  expansions  that  are  indefinite  in  their 
forms ;  and  some  have  quite  definitely-shaped  leaves.  Roots 
are  possessed  by  all  the  more  developed  genera  of  the  class ; 
but  there  are  other  genera,  as  Sphagnum,  which  have  no 
roots.  Here  the  fronds  are  thallus-like,  in  being  formed  of  only 
a  single  layer  of  cells ;  and  there  a  double  layer  gives  them 
a  more  leaf-like  character — a  difference  exhibited  between 
closely-allied  genera  of  one  order,  the  Mosses.  Equally  varied 
are  the  developments  of  the  foliar-organs  in  their  detailed 
structures :  now  being  without  mid-ribs  or  veins ;  now  having 
mid-ribs  but  no  veins  ;  now  having  both  mid-ribs  and  veins. 
Where  stem  and  leaves  exist,  their  imperfect  differentiation 
is  shown  by  the  fact,  that  in  many  cases  the  stem  is  covered 
by  an  epidermis  containing  stomata.  Nor  must  we  omit  the 
similarly-significant  circumstance,  that  whereas  in  the  lower 
Acrogens,  the  reproductive  elements  are  immersed  here  and 
there  in  the  thallus-like  frond  ;  they  are,  in  the  higher  orders, 
seated  in  well-specialized  and  quite  distinct  fructifying 
organs,  having  analogies  with  the  flowers  of  Phsenogams. 
Thus,  many  facts  imply  that  if  the  phsenogamic  type  is  to  be 
analyzed  at  all,  we  must  look  among  the  Acrogens  for  its  mor- 
phological components,  and  the  manner  of  their  integration. 
Already  we  have  seen  among  the  lower  Cryptogamia,  how 

*  Schleiden,  who  chooses  to  regard  as  an  axis,  that  which  Mr  Berkeley,  with 
more  obvious  truth,  calls  a  mid-rib,  says  : — "  The  flat  stem  of  the  Liverworts  pre- 
sents many  varieties,  consisting  frequently  of  one  simple  layer  of  thin-walled 
colls,  or  it  exhibits  in  its  axis  the  elements  of  the  ordinary  stem."  This  passage 
exemplifies  the  wholly  gratuitous  hypotheses  which  men  will  sometimes  espouse, 
to  escape  hypotheses  they  dislike.  Schleiden,  with  the  positiveness  characteristic 
of  him,  asserts  the  primordial  distinction  between  axial  organs  and  foliar  organs. 
In  the  higher  Acrogens,  he  sees  an  undeniable  stem.  In  the  lower  Acrogens,  clearly 
allied  to  them  by  their  fructification,  there  is  no  structure  having  the  remotest 
resemblance  to  a  stem.  But  to  save  his  hypothesis,  Schleiden  calls  that  "  a  flat 
stem,"  which  is  very  obviously  a  structure  in  which  stem  and  leaf  are  not  differ- 
entiated. He  is  the  more  to  be  blamed  for  this  unphilosophical  assumption,  since 
he  is  merciless  in  Vis  strictures  on  the  unphilosophical  assumptions  of  oth^r 
botanists. 

VOL.  II.  * 


48  MORPHOLOGICAL    DEVELOPMENT. 

as  they  become  integrated  and  definitely  limited,  aggregates 
acquire  the  habit  of  budding  out  other  aggregates,  on  reach- 
ing certain  stages  of  growth.  Cells  produce  other  cells 
endogenously  or  exogenously  ;  and  fronds  give  origin  to 
other  fronds  from  their  edges  or  surfaces.  We  have  seen,  too, 
that  the  new  aggregates  so  produced,  whether  of  the  first 
order  or  the  second  order,  may  either  separate  or  remain 
connected.  Fissiparously-multiplyiiig  cells  in  some  cases 
fly  asunder,  while  in  other  cases  they  unite  into  threads  or 
laminao  or  masses ;  and  fronds  originating  proliferously  from 
other  fronds,  sometimes  when  mature  disconnect  themselves 
from  their  parents,  and  sometimes  continue  attached  to  them. 
Whether  they  do  or  do  not  part,  is  clearly  determined  by 
their  nutrition.  If  the  conditions  are  such  that  they  can 
severally  thrive  better  by  separating  after  a  certain  develop- 
ment is  reached,  it  will  become  their  habit  then  to  separate ; 
since  natural  selection  will  favour  the  propagation  of  those 
which  separate  most  nearly  at  that  time.  If,  conversely,  it 
profits  the  species  for  the  cells  or  fronds  to  continue  longer 
attached,  which  it  can  only  do  if  their  growth  and  subse- 
quent powers  of  multiplication  are  thereby  increased ;  it  must 
happen,  through  the  continual  survival  of  the  fittest,  that 
longer  attachment  will  become  an  established  characteristic  ; 
and  by  persistence  in  this  process,  permanent  attachment 
will  result,  when  permanent  attachment  is  advantageous. 
That  disunion  is  really  a  consequence  of  relative  innu- 
trition, and  union  a  consequence  of  relative  nutrition, 
is  clear,  a  posteriori.  On  the  one  hand,  the  separation 
of  the  new  individuals,  whether  in  germs  or  as  developed 
aggregates,  is  a  decaying  away  of  the  connecting  tissue; 
and  this  implies  that  the  connecting  tissue  has  cease  1 
to  perform  its  function  as  a  channel  of  nutriment.  On 
the  other  hand,  where,  as  we  see  among  Phcenogams,  there 
is  about  to  take  place  a  separation  of  new  individuals  in 
the  shape  of  germs,  at  the  point  where  the  nutrition  is  the 
lowest,  a  sudden  increase  of  nutrition  will  cause  the  impend- 


THE    MORPHOLOGICAL   COMPOSITION    OF    PLANTS.  49 

ing  separation  to  be  arrested ;  and  the  fructifying  elements 
will  revert  towards  the  ordinary  form,  and  develop  in  con- 
nexion with  the  parent.  Turning  to  the  Acrogens,  wo 
•find  among  them,  many  indications  of  this  transition  from  dis- 
continuous development  to  continuous  development.    Thus  the 
Liverworts  give  origin  to  new  plants  by  cells  which  they 
throw  off  from  their  surfaces ;  as,  indeed,  we  have  seen  that 
much   higher  plants   do.     "According   to   Bischoff,"    says 
Schleiden,  "  both  the  cells  of  the  stem  (Jungermannia,  biden- 
tata)  and  those  of  the  leaves  (/.  cxsecta)  separate  themselves 
as  propagative  cells  from  the  plant,  and  isolated  cells  shoot 
out  and  develop  while  still  connected  with  the  parent  plant 
into  small  cellular  bodies  (J.  violaced),  which  separate  from 
the  plant,  and  grow  into  new  plants,  as  in  Mnium  androgynum 
among  the  Mosses."   Now  in  the  way  above  explained,  these 
propagative   cells   and   proliferous  buds,  may  continue  de- 
veloping in  connexion  with  the  parent,  to  various  degrees 
before  separating ;  or  the  buds  which  are  about  to  become 
fructifying  organs,  may  similarly,  under  increased  nutrition, 
develop  into  young  fronds.     As  Sir  ~W.  Hooker  says  of  the 
male  fructification  in  Jungermannia  furcata, — "  It  has  the  ap- 
pearance of  being  a  young  shoot  or  innovation  (for  in  colour 
and  texture  I  can  perceive  no  difference)  rolled  up  into  a 
spherical  figure."     On  finding  in  this  same  plant,  that  some- 
times the  proliferously-produced  frond,  buds  out  from  itself 
another  frond  before  separating  from  the  parent,  as  shown  in 
Fig.  46  ;  it  becomes  clear  that  this  long-continued  connexion, 
may  readily  pass   into  permanent  connexion.      And  when 
we  see  how,  even  among  Phaenogams,  buds  may  either  detach 
themselves  as  bulbils,  or  remain  attached  and  become  shoots  ; 
we  can  scarcely  doubt  that  among  inferior  plants,  less  de- 
finite in  their  modes  of  organization,  such  transitions  must 
continually  occur. 

Let  us  suppose,  then,  that  Fig.  73  is  the  frond  of  some 
primitive  Acrogen,  similar  in  general  characters  to  Junger- 
iiannia  epiphylla,  Fig.  43 ;  bearing,  like  it,  the  fructifying  buds 


50 


MORPHOLOGICAL   DEVELOPMENT. 


7* 


on  its  upper  surface,  and  haying  a  slightly- 
marked  mid-rib  and  rootlets.  And  sup- 
pose that,  as  shown,  a  second  aiy  frond  is 
proliferously  produced  from  the  mid-rib, 
and  continues  attached  to  it.  Evidently, 
the  ordinary  discontinuous  development, 
can  thus  become  a  continuous  development, 
only  on  condition  that  there  is  an  adequate 
supply,  to  the  secondary  frond,  of  such 
materials  as  are  furnished  by  the  rootlets : 
the  remaining,  materials  being  obtainable 
by  itself  from  the  air.  Hence,  that  portion 
of  the  mid-rib  lying  between  the  secondary 
frond  and  the  chief  rootlets,  having  its 
function  increased,  will  increase  in  bulk. 
An  additional  consequence  will  be,  a 
greater  concentration  of  the  rootlets — 
there  will  be  extra  growth  of  those  which 
are  most  serviceably  placed.  Observe,  next, 
that  the  structure  so  arising,  is  likely  to  be 
maintained.  Such  a  variation  implying, 
as  it  does,  circumstances  especially  favour- 
able to  the  growth  of  the  plant,  will  give 
to  the  plant  extra  chances  of  leaving  de- 
scendants ;  since  the  area  of  frond  sup- 
ported by  a  given  area  of  the  soil,  being 
greater  than  in  other  individuals,  there 
may  be  a  greater  production  of  spores.  And  then,  among 
the  more  numerous  descendants  thus  secured  by  it,  the  varia- 
tion will  give  advantages  to  those  in  which  it  recurs.  Such 
a  mode  of  growth  having,  in  this  manner,  become  established, 
let  us  ask  what  is  next  likely  to  result.  If  it  becomes  the 
habit  of  the  primary  frond  to  bear  a  secondary  frond  from  its 
mid-rib,  this  secondary  frond,  composed  of  physiological 
units  of  the  same  kind,  will  inherit  the  habit ;  and  supposing 


THE   MORPHOLOGICAL   COMPOSITION   OF   PLANTS.  51 

that  the  supply  of  mineral  matters  obtained  by  the  rootlets 
suffices  for  the  full  development  of  the  secondary  frond,  there 
is  a  likelihood  that  the  growth  from  it  of  a  tertiary  frond,  will 
become  an  habitual  characteristic  of  the  variety.  Along  with 
the  establishment  of  such  a  tertiary  frond,  as  shown  in  Fig. 
74,  there  must  arise  a  further  development  of  mid- rib  in  the 
primary  frond,  as  well  as  in  the  secondary  frond — a  develop- 
ment which  must  bring  with  it  a  greater  integration  of  the 
two  ;  while,  simultaneously,  extra  growth  will  take  place  in 
such  of  the  rootlets  as  are  most  directly  connected  with  this 
main  channel  of  circulation.  "Without  further  explanation  it 
will  be  seen,  on  inspecting  Figs.  75  and  76,  that  there  may 
in  this  manner  result  an  integrated  series  of  fronds,  placed 
alternately  on  opposite  sides  of  a  connecting  vascular  struc- 
ture. That  this  connecting  vascular  structure  will,  as  shown 
in  the  figures,  become  more  distinct  from  the  foliar  surfaces  as 
these  multiply,  is  no  unwarranted  assumption ;  for  we  have 
seen  in  compound-leaved  plants,  how,  under  analogous  con- 
ditions, mid-ribs  become  developed  into  separate  supporting 
parts,  which  acquire  some  of  the  characters  of  axes  while  as- 
suming their  functions.  And  now  mark  how  clearly 
the  structure  thus  built  up  by  integration  of  proliferously- 
growing  fronds,  corresponds  with  the  structure  of  the  more- 
developed  Jungermanniacece.  Each  of  the  fronds  successively 
produced,  repeating  the  characters  of  its  parent,  will  bear 
roots ;  and  will  bear  them  in  homologous  places,  as  shown. 
Further,  the  united  mid-ribs  having  but  very  little  rigidity, 
will  be  unable  to  maintain  an  erect  position.  Hence  there 
will  result  the  recumbent,  continuously-rooted  stem,  whicli 
these  types  exhibit.  Nay,  the  parallelism  is  more  complete 
than  the  figures  show.  To  avoid  confusion,  the  fronds  thus 
supposed  to  be  progressively  integrated,  have  been  repre- 
sented as  simple.  But,  as  shown  in  Fig.  45,  these  lower 
types  ordinarily  have  fronds  which  divide  dichotomously,  in 
such  way  that  one  division  is  larger  than  the  other  ;  and  thia 


52  MORPHOLOGICAL   DEVELOPMENT 

is  just  the  character  of  the  successive  leaves  in  the  higher 
types.  As  shown  in  Fig.  47,  each  leaf  is  usually  composed 
cf  two  unequal  lobes. 

A  natural  concomitant  of  the  mode  of  growth  here  de- 
scribed, is,  that  the  stem,  while  it  increases  longitudinally, 
increases  scarcely  at  all  transversely :  hence  the  name 
Acrogens.  Clearly  the  transverse  development  of  a  stem,  is 
the  correlative,  partly  of  its  function  as  a  channel  of  circula- 
tion, and  partly  of  its  function  as  a  mechanical  support. 
That  an  axis  may  lift  its  attached  leaves  into  the  air,  implies 
thickness  and  solidity  proportionate  to  the  mass  of  such 
leaves ;  and  an  increase  of  its  sap- vessels,  also  proportionate 
to  the  mass  of  such  leaves,  is  necessitated  when  the  roots 
are  all  at  one  end  and  the  leaves  at  the  other.  But  in  the 
generality  of  Acrogens,  these  conditions,  under  which  arises 
the  necessity  for  transverse  growth  of  the  axis,  are  absent, 
wholly  or  in  great  part.  The  stem  habitually  creeps  belov 
the  surface,  or  lies  prone  upon  the  surface  ;  and  where  it 
grows  in  a  vertical  or  inclined  direction,  does  this  by  at- 
taching itself  to  a  vertical  or  inclined  object.  Moreover, 
throwing  out  rootlets,  as  it  mostly  does,  at  intervals  through- 
out its  length,  it  is  not  called  upon  in  any  considerable  de- 
gree, to  transfer  nutritive  materials  from  one  of  its  ends  to 
the  other.  Hence  this  peculiarity  which  gives  their  name 
to  the  Acrogens,  is  a  natural  accompaniment  of  the  low 
degree  of  specialization  reached  in  them.  And  that  it  is  an 
incidental  and  not  a  necessary  peculiarity,  is  demonstrated 
by  two  converse  facts.  On  the  one  hand,  in  those  higher 
Acrogens  which,  like  the  tree-ferns,  lift  large  masses  of 
foliage  into  the  air,  there  is  just  as  decided  a  transverse  ex- 
pansion of  the  axis  as  in  Exogens.  On  the  other  hand,  in 
those  Exogens  which,  like  the  common  Dodder,  gain  sup- 
port and  nutriment  from  the  surfaces  over  which  they  creep, 
there  is  no  more  lateral  expansion  of  the  axis  than  is  habit- 
ual among  Acrogens.  Concluding,  as  we  are  thus  fully  justi- 
fied in  doing,  that  the  lateral  expansion  accompanying  longi- 


THE    MORPHOLOGICAL    COMPOSITION   OF   PLANTS.  53 

hulinal  extension,  which  is  a  general  characteristic  of 
Endogcns  and  Exogens  as  distinguished  from  Acrogens,  is 
nothing  more  than  a  concomitant  of  their  usually- vertical 
growth;*  let  us  now  go  on  to  consider  how  vertical  growth 
originates,  and  what  are  the  structural  changes  it  involves. 

§  193.  Plants  depend  for  their  prosperity  mainly  on  air 
and  light :  they  dwindle  where  they  are  smothered,  and 
thrive  where  they  can  expand  their  leaves  into  free  space 
and  sunshine.  Those  kinds  which  assume  prone  positions, 
consequently  labour  under  disadvantages  in  being  habitually 
interfered  with  by  one  another — they  are  mutually  shaded 
and  mutually  injured.  Such  of  them,  however,  as  happen, 
by  variations  in  mode  of  growth,  to  get  at  all  above  the  rest, 
are  more  Mkely  to  flourish  and  leave  offspring  than  the  rest. 
That  is  to  say,  natural  selection  will  favour  the  more  upright- 
growing  forms :  individuals  with  structures  that  lift  them 
above  the  rest,  are  the  fittest  for  the  conditions ;  and  by  the 
continual  survival  of  the  fittest,  such  structures  must  become 
established.  There  are  two  essentially-different  ways  in 
which  the  integrated  series  of  fronds  above  described,  may 
be  modified  so  as  to  acquire  the  stiffness  needful  for  main- 
taining perpendicularity.  "We  will  consider  them  separately. 

A  thin  layer  of  substance  gains  greatly  in  power  of  re- 
sisting a  transverse  strain,  if  it  is  bent  round  so  as  to  form  a 
tube — witness  the  difference  between  the  pliability  of  a  sheet 
of  paper  when  outspread,  and  the  rigidity  of  the  same  sheet 
of  paper  when  rolled  up.  Engineers  constantly  recognize 

*  I  am  indebted  to  Dr  Hooker  for  pointing  out  further  facts  supporting  this 
view.  In  his  Flora  Antarctica,  he  describes  the  genus  Lessonia  (see  Fig.  37)  and 
especially  L.  ovata,  as  having  a  mode  of  growth  simulating  that  of  the  Exogens. 
The  tall  vertical  stem  thickens  as  it  grows,  by  the  periodical  addition  of  layers 
to  its  periphery.  Among  lichens,  too,  it  seems  that  there  is  an  analogous  case. 
That  even  Thallogens  should  thus,  under  certain  conditions,  present  a  transversely- 
increasing  axis,  shows  that  there  is  nothing  absolute  in  the  character  which  gives 
the  names  to  the  two  highest  classes  of  plants,  in  contradistinction  to  the  clasi 
nearest  to  them. 


MORPHOLOGICAL   DEVELOPMKM'. 


77 


this  truth,  in  devising  appliances  by  which  the  greatest 
strength  shall  be  obtained  at  the  smallest  cost  of  material ; 
and  among  organisms,  we  see  that  natural  selection  habit- 
ually establishes  structures  conforming  to  the  same  principle, 
wherever  lightness  and  stiffness  are  to  be  combined.  The 
cylindrical  bones  of  mammals  and  birds,  and  the  holloAV 
shafts  of  feathers,  are  examples.  The  lower  plants,  too, 
furnish  cases  where  the  strength  needful  for  maintaining  an 
upright  position,  is  acquired  by  this  rolling  up  of  a  flat 
thallus  or  frond.  In  Fig.  77, 
we  have  an  Alga  which  ap- 
proaches towards  a  tubular 
distribution  of  substance ;  and 
which  has  a  consequent  rigid- 
ity. Sundry  common  forms 
of  lichen,  having  the  thallus 
folded  into  a  branched  tube, 
still  more  decidedly  display- 
ing the  connexion  between 
this  structural  arrangement 
and  this  mechanical  advantage.  And  from  the  particular 
class  of  plants  we  are  here  dealing  with — the  Acrogens — a 
type  is  shown  in  Fig.  78,  Riella  lielicophylla,  similarly  cha- 
racterized by  a  thin  frond  that  is  made  stiff  enough  to  stand, 
by  an  incurving  which,  though  it  does  not  produce  a  hollow 
cylinder,  produces  a  kindred  form.  If,  then,  as  we  have 
seen,  natural  selection  or  survival  of  the  fittest,  will  favour 
such  among  these  recumbent  Acrogens,  as  are  enabled,  by 
variations  of  their  structures,  to  maintain  raised  postures ; 
it  will  favour  the  formation  of  fronds  that  curve  round  upon 
themselves,  and  curve  round  upon  the  fronds  growing  out 
of  them.  What,  now,  will  be  the  result  should  such  a 
modification  take  place  in  the  group  of  proliferous  fronds 
represented  in  Fig.  76?  Clearly,  the  result  will  bo  a 
structure  like  that  shown  in  Fig.  79.  And  if  this  inrolling 
becomes  more  complete,  a  form  like  Junqwmannia  cordifolia* 


THE   MORPHOLOGICAL   COMPOSITION    OF   PLANTS.  55 

represented  in  Fig.  80,  will 
be  produced. 

When  the  successive  fronds 
are  thus  folded  round  so  com- 
pletely that  their  opposite 
edges  meet,  these  opposite 
edges  will  be  apt  to  unite  :  not 
that  they  will  grow  together 
after  being  formed,  but  that 
they  will  develop  in  connexion;  79 

or,  in  botanical  language,  will  become  "  adnate."  That  foliar 
surfaces  which,  in  their  embryonic  state,  are  in  close  contact, 
often  join  into  one,  is  a  familiar  fact.  It  is  habitually  so 
with  sepals  or  divisions  of  the  calyx.  In  all  campanulate 
flowers  it  is  so  with  petals.  And  in  some  tribes  of  plants 
it  is  so  with  stamens.  "We  are  therefore  well-warranted  in 
inferring,  that  under  the  conditions  above  described,  the  suc- 
cessive fronds  or  leaflets  will,  by  union  of  their  remote  edges, 
first  at  their  points  of  origin,  and  afterwards  higher  up, 
form  sheaths  inserted  one  within  another,  and  including  the 
axis.  This  incurving  of  the  successive  fronds, 

ending  in  the  formation  of  sheaths,  may  be  accompanied  by 
different  sets  of  modifications.  Supposing  Fig.  81  to  be  a 
transverse  section  of  such  a  type  (a  being  the  mid-rib,  and 
b  the  expansion  of  an  older  frond ;  while  c  is  a  younger  frond 
proliferously  developed  within  it),  there  may  begin  two  di- 
vergent kinds  of  changes,  leading  to  two  contrasted  struc- 
tures. If,  while  frond  continues  to  grow  out  of  frond,  the 
series  of  united  mid- ribs  continues  to  be  the  channel  of  circu- 
lation between  the  uppermost  fronds  and  the  roots — if,  as  a 
consequence,  the  compound  mid- rib,  or  rudimentary  axis,  con- 
tinues to  increase  in  size  laterally ;  there  will  arise  the  series 
of  transitional  forms  represented  by  the  transverse  sections 
82,  83,  84,  85 ;  ending  in  the  production  of  a  solid  axis, 
everywhere  wrapped  round  by  the  foliar  surface  of  tho 
frond,  as  an  outer  layer  or  sheath.  But  if,  on  the  other 


MORPHOLOGICAL    DEVELOPMENT. 


hand,  circumstances  favour  a  form  of  plant  which  maintains 
its  uprightness  at  the  smallest  cost    of  substance — if  the 


vascular  bundles  of  each  succeeding  mid-rib,  instead  of  re- 
maining concentrated,  become  distributed  all  round  the  tube 
formed  by  the  infolded  frond ;  then  the  structure  eventually 
reached,  through  the  transitional  forms  86,  87,  88,  89,  will 
be  a  hollow  cylinder.  And  now  observe  how  the 

two  structures  thus  produced,  correspond  with  two  kinds 
of  Endogens.  Fig.  90  represents  a  species  of  Dendrobium, 
in  which  we  see  clearly  how  each  leaf  is  but  a  continuation 
of  the  external  layer  of  a  solid  axis — a  sheath  such  as  would 
result  from  the  infolded  edges  of  a  frond  becoming  adnate ; 
and  on  examining  how  the  sheath  of  each  leaf  includes  the 
one  above  it,  and  how  the  successive  sheaths  include  the  axis, 
it  will  be  manifest  that  the  relations  of  parts  are  just  such 
as  exist  in  the  united  series  of  fronds  shown  in  Fig.  79 — the 
successive  nodes  answering  to  the  successive  points  of  origin 
of  the  fronds.  Conversely,  the  stem  of  a  grass,  Fig.  91,  dis- 
plays just  such  relations  of  parts,  as  would  result  from  the  de- 
velopment of  the  type  shown  in  Fig.  79,  if  instead  of  the  mid- 
ribs thickening  into  a  solid  axis,  the  matter  composing  them 
became  evenly  distributed  round  the  foliar  surfaces,  at  the 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  57 

name  time  that  the  incurved  edges  of  the  foliar  surfaces 
united.  The  arrangements  of  the  tubular  axis  and  its  ap- 
pendages, thus  resulting,  are  still  more  instructive  than  those 
of  the  solid  axis.  For  while,  even  more  clearly  than  in  the 
Dcndrobium,  we  see  at  the  point  b,  a  continuity  of  structure 
between  the  substance  of  the  axis  below  the  node,  and  the 
substance  of  the  sheath  above  the  node;  we  see  that  this 
sheath,  instead  of  having  its  edges  united  as  in  Dendrobium, 
has  them  simply  overlapping,  so  as  to  form  an  incomplete 
hollow  cylinder  which  may  be  taken  off  and  unrolled; 


and  we  see  that  were  the  overlapping  edges  of  this  sheath, 
united  all  the  way  from  the  node  a  to  the  node  b,  it  would 
constitute  a  tubular  axis,  like  that  which  precedes  it  or  like 
that  which  it  includes.  And  then,  giving  an  unexpected 
conclusiveness  to  the  argument,  it  turns  out  that  in  one 
family  of  grasses,  the  overlapping  edges  of  the  sheaths  do 
unite  :  thus  furnishing  us  with  a  demonstration  that  tubular 
structures  are  produced  by  the  incurving  and  joining  01 
foliar  surfaces ;  and  that  so,  hollow  axes  may  be  interpreted 
as  above,  without  making  any  assumption  unwarranted  by 
fact.  One  further  correspondence  between  the 

tj'pe  thus  ideally  constructed,  and  the  endogenous  type,  must 
oe  noted.  If,  as  already  pointed  out,  the  transverse  growth  of 


58  MORPHOLOGICAL    PEVELOPMEXT. 

an  axis  arises,  when  the  axis  conies  to  be  a  channel  of  circu- 
lation between  all  the  roots  at  one  of  its  extremities  and  all 
the  leaves  at  the  other ;  and  if  this  lateral  bulging  must  in- 
crease, as  fast  as  the  quantity  of  foliage  to  be  brought  in 
communication  with  the  roots  increases — especially  if  such 
foliage  has  at  the  same  time  to  be  raised  high  above  the 
earth's  surface  ;  what  must  happen  to  a  plant  constructed  in 
the  manner  just  described  ?  The  elder  fronds  or  foliar  or- 
gans, ensheathing  those  within  them,  as  well  as  the  incipient 
axis  serving  as  a  bond  of  union,  are  at  first  of  such  circum- 
ference only  as  suffices  to  inclose  these  undeveloped  parts. 
What,  then,  will  take  place  when  the  inclosed  parts  grow — • 
when  the  axis  thickens  while  it  elongates  ?  Evidently  the 
earliest-formed  sheaths,  not  being  large  enough  for  the 
swelling  axis,  must  burst ;  and  evidently  each  of  the  later- 
formed  sheaths  must,  in  its  turn,  do  the  like.  There  must 
result  a  gradual  exfoliation  of  the  successive  sheaths,  like 
that  indicated  as  beginning  in  the  above  figure  of  Dendro- 
tium ;  which,  at  a,  shows  the  bud  of  the  undeveloped  parts 
just  visible  above  the  enwrapping  sheaths,  while  at  b,  and  c, 
it  shows  the  older  sheaths  in  process  of  being  split  open. 
That  is  to  say,  there  must  result  the  mode  of  growth  which 
helps  to  give  the  name  Endogens  to  this  class. 

The  other  way  in  which  an  integrated  series  of  fronds 
may  acquire  the  rigidity  needful  for  maintaining  an  erect 
position,  has  next  to  be  considered.  If  the  successive  fronds 
do  not  acquire  such  habit  of  curling  as  may  be  taken  ad- 
Vantage  of  by  natural  selection,  so  as  to  produce  the  requisite 
stiflhess ;  then,  the  only  way  in  which  the  requisite  stiffness 
appears  producible,  is  by  the  thickening  and  hardening  of 
the  fused  series  of  mid-ribs.  The  incipient  axis  will  not,  in 
this  case,  be  inclosed  by  the  rolled-up  fronds ;  but  will  con- 
tinue exposed.  Survival  of  the  fittest  will  favour  the  genesis 
of  a  type,  in  which  those  portions  of  the  successive  mid- ribs 
that  enter  into  the  continuous  bond,  become  more  bulky  than 
the  disengaged  portions  of  the  mid- ribs :  the  individuals 


THE   MORPHOLOGICAL    COMPOSITION    OF    PLANTS. 


59 


which  thrive  and  have  the  best  chances  of  leaving  offspring, 
being,  by  the  hypothesis,  individuals  having  axes  stiff 
enough  to  raise  their  foliage  above  that  of  their  fellows 
At  the  same  time,  under  the  same  influences,  there  will  tend 
to  result  an  elongation  of  those  portions  of  the  mid-ribs, 
which  become  parts  of  the  incipient  axis  ;  seeing  that  it  will 
profit  the  plant  to  have  its  leaves  so  far  removed  from  one 
another,  as  to  prevent  mutual  interferences.  Hence,  from  the 
recumbent  type,  there  will  evolve,  by  indirect  equilibration, 
(§  167)  such  modifications  as  are  shown  in  Figs.  92,  93,  94  : 


the  first  of  which  is  a  slight  advance  on  the  ideal  type 
represented  in  Fig.  76,  arising  in  the  way  described ;  and 
the  others  of  which  are  actual  plants — Jungermannia  Hooltcii, 
and  J.  decipicns.  Thus  the  higher  Acrogens  show  us  how, 
along  with  an  assumption  of  the  upright  attitude,  there  does 
go  on,  as  we  see  there  must  go  on,  a  separation  of  the  leaf- 
producing  parts  from  the  root-producing  parts ;  a  greater 
development  of  that  connecting  portion  of  the  successive 
fronds,  by  which  they  are  kept  in  communication  with  the 
roots,  and  raised  above  the  ground ;  and  a  consequent  in- 
creased differentiation  of  such  connecting  portion  from  the 
parts  attached  to  it.  And  this  lateral  bulging  of  the  axis, 
directly  or  indirectly  consequent  on  its  functions  as  a  support 


60 


MORPHOLOGICAL   DEVELOPMENT. 


and  a  channel,  being  here  unrestrained  by  the  early-formed 
fronds  folded  round  it,  goes  on  without  the  bursting  of  these. 
Hence  arises  a  leading  character  of  what  is  called  exogenous 
growth — a  growth  which  is,  however,  still  habitually  accom- 
panied by  exfoliation,  in  flakes,  of  the  outermost  layer,  con- 
tinually being  cracked  and  split  by  the  accumulation  of 
layers  within  it.  And  now  if  we  examine  plants 

of  the  exogenous  type,  we  find  among  them  many  displaying 
the  stages  of  this  metamorphosis.  In  Fig.  95,  is  shown  a 
form  in  which  the  continuity  of  the  axis  with  the  mid- rib  of 
the  leaf,  is  manifest — a  continuity  that  is  conspicuous  in  the 
common  thistle.  Here  the  foliar  expansion,  running  some 
distance  down  the  axis,  makes  the  included  portion  of  the 


axis  a  part  of  its  mid-rib  ;  just  as  in  the  ideal  types  above 
drawn.  By  the  greater  growth  of  the  internodes,  which  are 
very  variable,  not  only  in  different  plants  but  in  the  same 
plant,  there  results  a  modification  like  that  delineated  in 
Fig.  96.  And  then,  in  such  forms  as  Fig.  97,  there  is  shown 
the  arrangement  that  arises  when,  by  more  rapid  develop- 
ment of  the  proximal  portion  of  the  mid-rib,  the  distal  part 
of  the  foliar  surface  is  separated  from  the  part  which  em- 
braces the  axis :  the  wings  of  the  mid-rib  still  serving,  how- 
ever, to  connect  the  two  portions  of  the  foliar  surface.  Such 
a  separation  is,  as  pointed  out  in  §  188,  an  habitual  occur- 
rence ;  and  in  some  compound  leaves,  an  actual  tearing  of  the 
inter- veinous  tissue,  is  caused  by  extra  growth  of  the  mid- rib. 
Modifications  like  this,  and  the  further  one  in  Fig.  98,  we 
may  expect  to  be  established  by  survival  of  the  fittest,  among 


TILE    MORPHOLOGICAL   COMPOSITION    OF    PLANTS.  61 

11  ose  plants  which  produce  considerable  masses  of  leavea ; 
Slice  the  development  of  mid-ribs  into  footstalks,  by  throw- 
ing the  leaves  further  away  from  the  axes,  will  diminish  the 
shading  of  the  leaves,  one  by  another.  And  then,  among 
plants  of  bushy  growth,  in  which  the  assimilating  surfaces 
become  still  more  liable  to  intercept  one  another's  light, 
natural  selection  will  continue  to  give  an  advantage  to  those 
which  carry  their  assimilating  surfaces  at  the  ends  of  the 
petioles,  and  do  not  develop  assimilating  surfaces  close  to 
the  axis,  where  they  are  most  shaded.  Whence  will  result 
a  disappearance  of  the  stipules  and  the  foliar  fringes  of  the 
mid-ribs  ;  ending  in  the  production  of  the  ordinary  stalked 
leaf,  Fig.  99,  which  is  characteristic  of  trees.  Meanwhile, 
the  axis  thickens  in  proportion  to  the  number  of  leaves  it 
has  to  carry,  and  to  put  in  communication  with  the  roots  ; 
and  so  there  comes  to  be  a  more  marked  contrast  between  it 
and  the  reticles,  severally  carrying  a  leaf  each.* 

§  194.  When,  in  the  course  of  the  process  above  sketched 
out,  there  has  arisen  such  community  of  nutrition  among  the 
fronds  thus  integrated  into  a  series,  that  the  younger  ones 
are  aided  by  materials  which  the  older  ones  have  elaborated ; 
the  younger  fronds  will  begin  to  show,  at  earlier  and  earlier 
periods  of  development,  the  structures  about  to  originate 
from  them.  Abundant  nutrition  will  abbreviate  the  intervals 
between  the  successive  prolifications ;  so  that  eventually, 
while  each  frond  is  yet  imperfectly  formed,  the  rudiment  of 
the  next  will  begin  to  show  itself.  All  embryology  justifies 
this  inference.  The  analogies  it  furnishes  lead  us  to  expect 
that  when  this  serial  arrangement  becomes  organic,  the 
growing  part  of  the  series  will  show  the  general  relations  of 

*  Since  tbii  paragraph  was  put  in  type,  I  have  observed  that  in  some  varieties 
of  Cineraria,  as  probably  in  other  plants,  a  single  individual  furnishes  all  these 
forms  of  leaves— all  gradations  between  xinstipulated  leaves  on  long  petioles,  and 
leaves  that  embrace  the  axis.  It  may  be  added  that  the  distribution  of  these  va- 
rious forms,  is  quite  in  harmony  with  the  rationale  above  given. 


82  MORPHOLOGICAL   DEVELOPMENT. 

the  forthcoming  parts,  while  they  are  very  small  and  un« 
specialized.  What  will  in  such  case  be  the  appearances  they 
assumed  ?  We  shall  have  no  difficulty  in  perceiving  what  it 
will  be,  if  we  take  a  form  like  that  shown  in  Fig.  92,  and 
dwarf  its  several  parts  at  the  same  time  that  we  generalize 
them.  Figs.  100,  101,  102,  and  103,  will  show  the  result ; 
and  in  Fig.  104,  which  is  the  bud  of  an  exogen,  we  see  how 


*>4\     1 

clear  is  the  morphological  correspondence :  a  being  the 
rudiment  of  a  foliar  organ  beginning  to  take  shape  ;  b  being 
the  almost  formless  rudiment  of  the  next  foliar  organ  ;  and 
c  being  the  quite-undifferentiated  part  whence  the  rudiments 
of  subsequent  foliar  organs  are  to  arise. 

And  now  we  are  prepared  for  entering  on  a  still-remaining 
question  respecting  the  structure  of  Phaenogams — what  is  the 
origin  of  axillary  buds  ?  As  the  synthesis  at  present  stands, 
it  does  not  account  for  these ;  but  on  looking  a  little  more 
closely  into  the  matter,  we  shall  find  that  the  axillary  buds 
are  interpretable  in  the  same  manner  as  the  terminal  buds. 
So  to  interpret  them,  however,  we  must  return  to  that  pro- 
cess of  proliferous  growth  with  which  we  set  out,  for  the  pur- 
pose of  observing  some  facts  not  before  named.  Delcsseria 
Injpoglossum,  Fig.  105,  represents  a  seaweed  of  the  same  genus 
as  one  outlined  in  Fig.  40  ;  but  of  a  species  in  which  pro- 
liferous growth  is  carried  much  further.  Here,  not  only  does 
the  primary  frond  bud  out  many  secondary  fronds  from  ita 
mid- rib  ;  but  most  of  the  secondary  fronds  similarly  bud  out 
several  tertiary  fronds;  and  even  by  some  of  the  tertiary 
fronds,  this  prolification  is  repeated.  Besides  being  shown 
that  the  budding  out  of  several  fronds  from  one  frond,  may 
become  habitual ;  we  are  also  shown  that  it  may  become  a 
habit  inherited  by  the  fronds  so  produced,  and  also  by  the 


Tine    MORPHOLOGICAL    COMPOSITION    OF    PLANTS. 


03 


fronds  they  produce :  the  manifestation  of  the  tendency, 
being  probably  limited  only  by  failure  of  nutrition.  That 
under  fit  conditions,  an  analogous  mode  of  growth  will  occur 
in  fronds  of  the  acrogenic  type,  like  those  we  set  out  with,  is 
shown  by  the  case  of  Jungermannia  furcata,  Figs.  45,  46,  in 
which  such  compound  prolification  is  partially  displayed. 
Lot  us  suppose  then,  that  the  frond  a,  Fig.  106,  produces 


not  only  a  single  secondary  frond  b,  but  also  another  such 
secondary  frond,  I'.  Let  us  suppose,  further,  that  the  frond 
b  is  in  like  manner  doubly  proliferous :  producing  both  c 
and  c'.  Lastly,  let  us  suppose  that  in  the  second  frond  /;' 
which  a  produces,  as  well  as  in  the  second  frond  c'  which  b 
produces,  the  doubly-proliferous  habit  is  manifested.  If, 
now,  this  habit  grows  organic — if  it  becomes,  as  it  natur- 
ally will  become,  the  characteristic  of  a  plant  of  luxuriant 
growth,  the  unfolding  parts  of  which  can  be  fed  by  the  un- 
folded parts ;  it  will  happen  with  each  lateral  series,  as  with 
the  main  series,  that  its  successive  components  will  begin  to 
shew  themselves  at  earlier  and  earlier  stages  of  development. 
And  in  the  same  way  that,  by  dwarfing  and  generalizing 


54  MOIU'HOLOGICAJ,    DEVELOPMENT. 

the  original  series,  we  arrive  at  a  structure  like  that  of  the 
terminal  bud ;  by  dwarfing  and  generalizing  a  lateral  series, 
as  shown  in  Figs.  107 — 110,  we  arrive  at  a  structure  an- 
swering in  nature  and  position  to  the  axillary  bud. 


Facts  confirming  these  interpretations,  are  afforded  by 
the  structure  and  distribution  of  buds.  The  phoenogamic 
axis  in  its  primordial  form,  being  an  integrated  series  of 
folia  ;  and  the  development  of  that  part  by  which  these  folia 
are  held  together  at  considerable  distances  from  one  another, 
taking  place  afterwards ;  it  is  inferable  from  the  general 
principles  of  embryology,  that  in  its  rudimentary  stages,  the 
phsenogamic  axis  will  have  its  foliar  parts  much  more  clearly 
marked  out  than  its  axial  parts.  This  we  see  in  every  bud. 
Every  bud  consists  of  the  rudiments  of  leaves  packed  to- 
gether without  any  appreciable  internodal  spaces ;  and  the 
internodal  spaces  begin  to  increase  with  rapidity,  only  when 
the  foliar  organs  have  been  considerably  developed.  More- 
over, where  nutrition  is  defective,  and  arrest  of  development 
takes  place — that  is,  where  a  flower  is  formed — the  inter- 
nodes  remain  undeveloped :  the  process  of  unfolding  ceases 
before  the  later-acquired  characters  of  the  phaenogamic  axis 
are  assumed.  Lastly,  as  the  hypothesis  leads  us  to  expect, 
axillary  buds  make  their  appearances  later  than  the  foliar 
organs  which  they  accompany ;  and  where,  as  at  the  ends  of 
axes,  these  foliar  organs  show  failure  of  chlorophyll,  the 
axillary  buds  are  not  produced  at  all.  That  these  are  in- 
ferable traits  of  structure,  will  be  manifest  on  contemplating 
Figs.  106 — 110 ;  and  on  observing,  first,  that  the  doubly- 
proliferous  tendency  of  which  the  axillary  bud  is  a  result,  im- 
plies abundant  nutrition ;  and  on  observing,  next,  that  the 
original  place  of  secondary  prolification,  is  such  that  the  foliar 


THE   MORPHOLOGICAL   COMPOSITION    OF    PLANTS.  65 

surface  on  which,  it  occurs,  must  grow  to  some  extent  before 
the  bud  appears. 

On  thus  looking  at  the  matter — on  contemplating  afresh 
the  ideal  type  shown  in  Fig.  108,  and  noting  how,  by  the 
conditions  of  the  case,  the  secondary  prolifications  must  cease 
before  that  primary  prolification  which  produces  the  main 
axis ;  we  are  enabled  to  reconcile  all  the  phenomena  of  axil- 
lary gemmation.  "We  see  harmony  among  the  several  facts — 
first,  that  the  axillary  bud  becomes  a  lateral,  leaf-bearing 
axis  if  there  is  abundant  material  for  growth  ;  second,  that 
its  development  is  arrested,  or  it  becomes  a  flower-bearing 
axis,  if  the  supply  of  sap  is  but  moderate ;  third,  that  it  is 
absent  when  the  nutrition  is  failing.  We  are  no  longer 
committed  to  the  gratuitous  assumption,  that  in  the  phaeno- 
gamic  type,  there  must  exist  an  axillary  bud  to  each  foliar 
organ ;  but  we  are  led  to  conclude,  a  priori,  that  which  we 
find,  a  posteriori,  that  axillary  buds  are  as  normally  absent 
in  flowers  as  they  are  normally  present  lower  down  the 
axis.  And  then,  to  complete  the  argument,  we  are  prepared 
for  the  corollary  that  axillary  prolification  may  naturally 
arise  even  at  the  ends  of  axes,  provided  the  failing  nutrition 
which  causes  the  dwarfing  of  the  foliar  organs  to  form  a 
flower,  be  suddenly  changed  into  such  high  nutrition  as  to 
transform  the  components  of  the  flower  into  appendages 
that  are  green,  if  not  otherwise  leaf-like — a  condition  under 
which  only,  this  phenomenon  is  proved  to  occur. 

§  195.  One  more  question  presents  itself,  when  we  con- 
trast the  early  stages  of  development  in  the  two  classes  of 
Phsenogams ;  and  a  further  answer  supplied  by  the  hypothe- 
sis, gives  to  the  hypothesis  a  further  probability.  It  is  cha- 
racteristic of  an  endogen,  to  have  a  single  seed-leaf  or  coty- 
ledon ;  and  it  is  characteristic  of  an  exogen,  to  have  at  least 
two  cotyledons,  if  not  more  than  two.  That  is  to  say,  the 
monocotyledonous  mode  of  germination  everywhere  co- 
exists with  the  endogenous  mode  of  growth ;  and  along  with 


66  MORPHOLOGICAL   DEVELOPMENT. 

the  exogenous  mode  of  growth,  there  always  goes  either  a 
dicotyledonous  or  polycotyledonous  germination.  Why  is 
this?  Such  correlations  cannot  be  accidental— cannot  be 
meaningless.  A  true  theory  of  the  phgenogamic  types,  in 
their  ori°in  and  divergence,  should  account  for  the  connex- 
ion of  these  traits.  Let  us  see  whether  the  foregoing  theory 
docs  this. 

The  higher  plants,  like  the  higher  animals,  bequeath  to 
their  offspring  more  or  less  of  nutriment  and  structure. 
Superior  organisms  of  either  kingdom  do  not,  as  do  all  in- 
ferior organisms,  cast  off  their  progeny  in  the  shape  of 
minute  portions  of  protoplasm,  unorganized  and  without 
stocks  of  material  fit  for  them  to  organize ;  but  they  either 
deposit  along  with  the  germs  they  cast  off,  certain  quantities 
of  albumenoid  substance,  fit  for  them  to  appropriate  while 
they  develop  themselves,  or  else  they  continue  to  supply  such 
substance  while  the  germs  partially-develop  themselves  before 
their  detachment.  Among  plants,  this  constitutes  the  dis- 
tinction between  seeds  and  spores.  Every  seed  contains  a 
store  of  food  to  serve  the  young  plant  during  the  first  stages 
of  its  independent  life ;  and  usually,  too,  before  the  seed  is 
detached,  the  young  plant  is  so  far  advanced  in  structure, 
that  it  bears  to  the  attached  stock  of  nutriment  much  the 
same  relation  that  the  young  fish  bears  to  the  appended  yelk- 
bag  at  the  time  of  leaving  the  egg.  Sometimes,  indeed,  the 
development  of  chlorophyll  gives  the  seed-leaves  a  bright 
green,  while  the  seed  is  still  contained  in  the  parent- 
pod.  This  early  organization  of  the  phoeno- 
gam,  must  be  supposed  rudely  to  indicate  the  type  out  of 
which  the  pha^nogamic  type  arose.  On  the  foregoing  hypo- 
thesis, the  seed-leaves  therefore  represent  the  primordial 
fronds — which,  indeed,  they  simulate  in  their  simple,  cellular, 
unveined  structures.  And  the  question  here  to  be  asked  is — 
do  the  different  relations  of  the  parts  in  young  endogens  and 
exogens  correspond  with  the  different  relations  of  the  primor- 
dial fronds,  severally  implied  by  the  endogenous  and  the 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  67 

exogenous  modes  of  growth  ?  "We  shall  find  that  they  do. 
Starting,  as  before,  with  the  proliferous  form  shown  in 
Fig.  Ill,  it  is  clear  that  if  the  strength  required  for  main- 
taining the  vertical  attitude,  is  obtained  by  the  rolling  up  of 
the  fronds,  the  primary  frond  will  more  and  more  conceal  the 
secondary  frond  within  it.  At  the  same  time,  the  secondary 
frond  must  continue  to  be  dependent  on  the  first  for  its  nutri- 
tion ;  and  being  produced  within  the  first,  must  be  prevented 
by  defective  supply  of  light  and  air,  from  ever  becoming  syn- 
chronous in  its  development  with  the  first.  Hence,  this 
infolding  which  leads  to  the  endogenous  mode  of  growth, 
implies  that  there  must  always  continue  such  pre-eminence 


of  the  first-formed  frond  or  its  representative,  as  to  make  the 
germination  monocotyledonous.  Figs.  Ill  to  115,  show  the 
transitional  forms  that  would  result  from  the  infolding  of 
the  fronds.  In  Fig.  116,  a  vertical  section  of  the  form  repre- 
sented in  Fig.  115,  are  exhibited  the  relations  of  the  succes- 


68  MORPHOLOGICAL   DEVELOPMENT. 

sive  fronds  to  each  other.  The  modified  relations  that  would 
result,  if  the  nutrition  of  the  embryo  admitted  of  anticipatory 
development  of  the  successive  fronds,  is  shown  in  Fig.  117. 
And  how  readily  the  structure  may  pass  into  that  of  the 
monocotyledoiious  germ,  will  be  seen  on  inspecting  Fig.  118 ; 
which  is  a  vertical  section  of  an  actual  monocotyledon  at  an 
early  stage — the  incomplete  lines  at  the  left  of  its  root,  indi- 
cating its  connexion  with  the  seed.*  Contrariwise, 
where  the  strength  required  for  maintaining  an  upright  atti- 
tude is  not  obtained  by  the  rolling  up  of  the  fronds,  but  by 
the  strengthening  of  the  continuous  mid- rib,  the  second 
frond,  so  far  from  being  less  favourably  circumstanced  than 
the  first,  becomes  in  some  respects  even  more  favourably 
circumstanced :  being  above  the  other,  it  gets  a  greater  share 
of  light,  and  it  is  less  restricted  by  surrounding  obstacles. 
There  is  nothing,  therefore,  to  prevent  it  from  rapidly  gaining 
an  equality  with  the  first.  And  if  we  assume,  as  the  truths  of 
embryology  entitle  us  to  do,  an  increasing  tendency  towards 
anticipation  in  the  development  of  subsequent  fronds — if 
we  assume  that  here,  as  in  other  cases,  structures  which 
were  originally  produced  in  succession,  will,  if  the  nutrition 
allows  and  no  mechanical  dependence  hinders,  come  to  be  pro 
duced  simultaneously ;  there  is  nothing  to  prevent  the  pas- 
sage of  the  type  represented  in  Fig.  Ill,  into  that  represented 

*  Since  these  figures  were  put  on  the  block,  it  has  occurred  to  me  that  the 
relations  would  be  still  clearer,  were  the  primary  frond  represented  as  not  taking 
part  in  these  processes  of  modification,  which  have  been  described  as  giving  rise 
to  the  erect  form ;  as,  indeed,  the  rooting  of  its  under  surface  will  prevent  it  from 
doing  in  any  considerable  degree.  In  such  case,  each  of  the  Figs.  Ill  to  117, 
should  have  a  horizontal  rooted  frond  at  its  base,  homologous  with  the  pro-em- 
bryo among  Acrogens.  This  primary  frond  would  then  more  manifestly  stand  in 
the  same  relation  to  the  rest,  as  the  cotyledon  does  to  the  plumule — both  by 
position,  and  as  a  supplier  of  nutriment.  Fig.  117  a,  which  I  am  enabled  to 
add,  shows  that  this  would  complete  the  interpretation.  Of  the  dicotyledonous 
series,  it  is  needful  to  add  no  further  explanation  than  that  the  difference  in  habit 
of  growth,  will  permit  the  second  frond  to  root  itself  as  well  as  the  first ;  and  so 
to  become  an  additional  source  of  nutrition,  similarly  circumstanced  to  the  first 
and  equal  with  it. 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  69 

in  Fig.  122.  Or  rather,  there  is  everything  to  facilitate  it ; 
seeing  that  natural  selection  will  continually  favour  the  pro- 
duction of  a  form  in  which  the  second  frond  grows  in  such 
way  as  not  to  shade  the  first,  and  in  such  way  as  allows  the 
axis  readily  to  assume  a  vertical  position. 

Thus,  then,  is  interpretable  the  universal  connexion  between 
monocotyledonous  germination  and  endogenous  growth;  as 
well  as  the  similarly-universal  connexion  between  exogenous 
growth  and  the  development  of  two  or  more  cotyledons. 
That  it  explains  these  fundamental  relations,  adds  very 
greatly  to  the  probability  of  the  hypothesis. 

§  196.  While  we  are  in  this  manner  enabled  to  discern 
the  kinship  that  exists  between  the  higher  vegetal  types 
themselves,  as  well  as  between  them  and  the  lower  types  ;  we 
are  at  the  same  time  supplied  with  a  rationale  of  those  truths 
which  vegetal  morphologists  have  established.  Those  homo- 
logics  which  "Wolff  indicated  in  their  chief  outlines  and 
Groethe  followed  out  in  detail,  have  a  new  meaning  given  to 
them  when  we  regard  the  phcenogamic  axis  as  having  been 
evolved  in  the  way  described.  Forming  the  modified  con- 
ception which  we  are  here  led  to  do,  respecting  the  units  of 
which  a  flowering  plant  is  composed,  we  are  no  longer  left 
without  an  answer  to  the  question — What  is  an  axis  ?  And 
we  are  helped  to  understand  the  naturalness  of  those  cor- 
respondences which  the  successive  members  of  each  shoot 
display.  Let  us  glance  at  the  facts  from  our  present  stand- 
point. 

The  unit  of  composition  of  a  Phaenogam,  is  such  portion  of 
a  shoot  as  answers  to  one  of  the  primordial  fronds.  This 
portion  is  neither  one  of  the  foliar  appendages  nor  one  of  the 
internodes ;  but  it  consists  of  a  foliar  appendage  together 
with  the  preceding  internode,  including  the  axillary  bud 
where  this  is  developed.  The  parts  intercepted  by  the  dotted 
lines  in  Fig.  123,  constitute  such  a  segment ;  and  the  true 
homology  is  between  this  and  any  other  foliar  organ  with  the 


70  MORPHOLOGICAL    DEVELOPMENT. 

portion  of  the  axis  below  it.  And  now  observe  how,  when  we 
take  this  for  the  unit  of  composition,  the  metamorphoses 
which  the  phtenogamic  axis  displays,  are  inferable  from  known 
laws  of  development.  Embryology  teaches  us  that  arrest 

of  development  shows  itself  first  in  the  absence  of  those  parts 
that  have  arisen  latest  in  the  course  of  evolution ;  that  if 
defect  of  nutrition  causes  an  earlier  arrest,  parts  that  are  of 
more  ancient  origin  abort ;  and  that  the  part  alone  produced 
when  the  supply  of  materials  fails  near  the  outset,  is  the  prim- 
ordial part.  "We  must  infer,  therefore,  that  in  each  seg- 
ment of  a  PhaBnogam,  the  foliar  organ,  which  answers  to  the 
primordial  frond,  will  be  the  most  constant  element ;  and 
that  the  internode  and  the  axillary  bud,  will  be  successively 
less  constant.  This  we  find.  Along  with  a  smaller  size  of 
foliar  surface  implying  lower  nutrition,  it  is  usual  to  see  a 
much- diminished  internode  and  a  less-pronounced  axillary 
bud,  as  in  Fig.  124.  On  approaching  the  flower,  the 

\ 


axillary  bud  disappears ;  and  the  segment  is  reduced  to 
a  small  foliar  surface,  with  an  internode  which  is  in  most 
cases  very  short  if  not  absent,  as  in  125  and  126.  In  the 
flower  itself,  axillary  buds  and  internodes  are  both  want- 
ing: there  remains  only  a  foliar  surface  (127),  which, 
though  often  larger  than  the  immediately  preceding  foliar 
surface,  shows  failing  nutrition  by  absence  of  chlorophyll. 
And  then,  in  the  quite  terminal  organs  of  fructification  (129), 
we  have  the  foliar  part  itself  reduced  to  a  mere  rudiment. 
Though  these  progressive  degenerations  are  by  no  means 
regular,  being  in  many  cases  varied  by  adaptation  to  par- 
ticular requirements,  yet  it  cannot,  I  think,  be  questioned, 


THE   MORPHOLOGICAL   COMPOSITION    OF    PLANTS.  71 

that  the  general  relations  are  as  described,  and  that  they  are 
such  as  the  hypothesis  leads  us  to  expect.  Nor  are 

we  without  a  kindred  explanation  of  certain  remaining  traita 
of  foliar  organs  in  their  least-developed  forms.  Petals, 
stamens,  pistils,  &c.,  besides  reminding  us  of  the  primordial 
fronds  by  their  diminished  sizes,  and  by  the  want  of  those 
several  supplementary  parts  which  the  preceding  segments 
possess,  also  remind  us  of  them  by  their  histologictil  charac- 
ters :  they  consist  of  simple  cellular  tissue,  scarcely  at  all 
differentiated.  The  fructifying  cells,  too,  which  here  make 
their  appearance,  are  borne  in  ways  like  those  in  which  the 
lower  Aero  gens  bear  them — at  the  edge  of  the  frond,  or  at 
the  end  of  a  peduncle,  or  immersed  in  the  general  substance  ; 
as  in  Figs.  128  and  129.  Nay,  it  might  even  be  said  that 
the  colours  assumed  by  these  terminal  folia,  call  to  mind  the 
plants  out  of  which  we  conclude  that  Phsanogams  have  been 
evolved ;  for  it  is  said  of  the  fronds  of  the  Jungcrmanniaccce, 
that  "  though  under  certain  circumstances  of  a  pure  green, 
they  are  inclined  to  be  shaded  with  red,  purple,  chocolate,  or 
other  tints." 

As  thus  understood,  then,  the  homologies  among  the  parts 
of  the  pha3nogamic  axis  are  interpretable,  rot  as  due  to  a 
needless  adhesion  to  some  typical  form  or  fulfilment  of  a  pre- 
determined plan ;  but  as  the  inevitable  consequences  of  the 
mode  in  which  the  phaBnogamic  axis  originates. 

§  197.  And  now  it  remains  only  to  observe,  in  confirmation 
of  the  foregoing  synthesis,  that  it  at  once  explains  for  us 
various  irregularities.  When  we  see  leaves  sometimes  pro- 
ducing leaflets  from  their  edges  or  extremities,  we  recognize 
in  the  anomaly,  a  resumption  of  an  original  mode  of  growth  : 
fronds  frequently  do  this.  When  we  learn  that  a  flowering 
plant,  as  the  Drosera  intermedia,  has  been  known  to  develop 
a  young  plant  from  the  surface  of  one  of  its  leaves,  we  are  ut 
once  reminded  of  the  proliferous  growths  and  fructifying 
organs  in  the  Liverworts,  The  occasional  production  of  bul- 
VOL.  II,  4 


/2  MOUPHOLOGICAL  DEVELOPMENT. 

bils  by  Phtenogams,  ceases  to  be  so  surprising  when  we  find 
it  to  be  habitual  among  the  inferior  Acrogens ;  and  when  wa 
see  that  it  is  but  a  repetition,  on  a  higher  stage,  of  that  self- 
detachment  which  is  common  among  proliferously-produced 
fronds.  Nor  are  we  any  longer  without  a  solution  of  that 
transformation  of  foliar  organs  into  axial  organs,  which 
not  uncommonly  takes  place.  How  this  last  irregularity 
of  development  is  to  be  accounted  for,  we  will  here  pause  a 
moment  to  consider.  Let  us  first  glance  at  our  data. 

The  form  of  every  organism,  we  have  seen,  must  depend 
cjn  the  structures  of  its  physiological  units.  Any  group  of 
such  physiological  units  will  tend  to  arrange  itself  into  the 
complete  organism,  if  it  is  uncontrolled  and  placed  in  fit 
conditions.  Hence  the  development  of  fertilized  germs ;  and 
hence  the  development  of  those  self-detached  cells  which 
characterize  some  plants.  Conversely,  physiological  units 
which  form  a  small  group  involved  in  a  larger  group,  and  are 
subject  to  all  the  forces  of  the  larger  group,  will  become  sub- 
ordinate in  their  structural  arrangements  to  the  larger  group 
— will  be  co-ordinated  into  a  part  of  the  major  whole,  in- 
stead of  co-ordinating  themselves  into  a  minor  whole.  This 
antithesis  will  be  clearly  understood  on  remembering  how, 
on  the  one  hand,  a  small  detached  part  of  a  hydra  soon 
moulds  itself  into  the  shape  of  an  entire  hydra ;  and  how, 
on  the  other  hand,  the  cellular  mass  that  buds  out  in  place 
of  a  lobster's  lost  claw,  gradually  assumes  the  form  of  a  claw 
— has  its  parts  so  moulded  as  to  complete  the  structure  of 
the  organism :  a  result  which  we  cannot  but  ascribe  to  the 
forces  which  the  rest  of  the  organism  exerts  upon  it.  Con- 
sequently, among  plants,  we  may  expect  that  whether  any 
portion  of  protoplasm  moulds  itself  into  the  typical  form 
around  an  axis  of  its  own,  or  is  moulded  into  a  part  subor- 
dinate to  another  axis,  will  depend  on  the  rela'ive  mass  of 
its  physiological  units — the  accumulation  of  them  that  has 
taken  place  before  the  assumption  of  any  structural  arrange- 
ment. A  few  illustrations  will  make  clear  the  validity  of 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS. 


73 


this  inference.  In  the  compound  leaf,  Fig.  65,  the  several 
lateral  growths  a,  b,  c,  d,  are  manifestly  homologous  ;  and 
on  comparing  a  number  of  such  leaves  together,  it  will  be 
seen  that  one  of  these  lateral  growths  may  assume  any  de- 
gree of  complexity,  according  to  the  degree  of  its  nutrition. 
Every  fern  leaf  exemplifies  the  same  general  truth  still  bet- 
ter. "Whether  each  sub-frond  remains  an  undeveloped  wing 
of  the  main  frond,  or  whether  it  organizes  itself  into  a  group 
of  frondlets  borne  by  a  secondary  rib,  or  whether,  going 
further,  as  it  often  does,  it  gives  rise  to  tertiary  ribs,  is 
clearly  determined  by  the  supply  of  materials  for  growth  ; 
since  such  higher  developments  are  habitually  most  marked 
at  points  where  the  nutrition  is  greatest  ;  namely,  next  the 
stem.  But  the  clearest  evidence  is  afforded  among  the  Alga-. 
which,  not  drawing  nutriment  from  roots,  have  their  pa^ts 
much  less  mutually  dependent  ;  and  are  therefore  capable  of 
showing  more  clearly,  how  any  part  may  remain  an  append- 
age or  may  become  the  parent  of  appendages,  according  to 
circumstances.  In  the  annexed  Fig.  130, 
representing  a  branch  of  Ptilota  plumosa, 
we  see  how  a  wing  grows  into  a  wing-bear- 
ing branch,  if  its  nutrition  passes  a  certain 
point.  This  form,  so  strikingly  like  that  of 
the  feathery  crystallizations  of  many  inor- 
ganic substances,  proves  to  us  that,  as  in 
such  crystallizations,  the  simplicity  or  com- 
plexity of  structure  at  any  place,  depends 
on  the  quantity  of  matter  that  has  to  be 
polarized  at  that  place  in  a  given  time.* 


*  How  the  element  of  time  modifies  the  result,  is  shown  by  the  familiar  fact  that 
crystals  rapidly  formed  are  small ;  and  that  they  become  larger  when  they  are 
formed  more  slowly.  If  the  quantity  of  molecules  contained  in  a  solution  is  rela- 
tively great,  so  that  the  mutual  polarities  of  the  molecules  crowded  together  in 
every  place  throughout  the  solution  are  intense,  there  arises  a  crystalline  aggre- 
gation around  local  axes  ;  whereas,  in  proportion  as  the  local  action  of  molecules 
on  one  another  is  rendered  less  intense  by  their  wider  dispersion,  they  become 


74  MOKI'HOLOGICAL    DEVELOPMENT. 

Hence,  then,  we  are  not  without  an  interpretation  of  those 
over-developments  which  the  phrcnogamic  axis  occasionally 
undergoes.  Fig.  104,  represents  the  phaenogamic  hud  in  its 
rudimentary  state.  The  lateral  process  6,  which  ordinarily 
becomes  a  foliar  appendage,  differs  very  little  from  the 
terminal  process  c,  which  is  to  become  an  axis — differs 
mainly  in  having,  at  this  period  when  its  form  is  being 
determined,  a  smaller  bulk.  If  while  thus  undifferentiated, 
its  nutrition  remains  inferior  to  that  of  the  terminal  process, 
it  becomes  moulded  into  a  part  that  is  subordinate  to  the 
general  axis.  But  if,  as  sometimes  happens,  there  is  supplied 
to  it  such  an  abundance  of  the  materials  needful  for  growth, 
that  it  becomes  as  large  as  the  terminal  process ;  then  we 
may  naturally  expect  it  to  begin  moulding  itself  round  an 
axis  of  its  own :  a  foliar  organ  will  be  replaced  by  an  axial 
organ.  And  this  result  will  be  especially  liable  to  occur, 
when  the  growth  of  the  axis  has  been  previously  under- 
going that  arrest  which  leads  to  the  formation  of  a  flower ; 
that  is,  Arhen,  from  defect  of  materials,  the  terminal  process 
has  almost  ceased  to  increase,  and  when  some  concurrence  of 
favourable  causes,  brings  a  sudden  access  of  sap,  which  reaches 
the  lateral  processes  before  it  reaches  the  terminal  process. 

§  198.  The  general  conclusion  to  which  these  various  lines 
of  evidence  converge,  is,  then,  that  the  shoot  of  a  flowering 
plant  is  an  aggregate  of  the  third  degree  of  composition. 
Taking  as  aggregates  of  the  first  order,  those  small  masses 
of  protoplasm  which  ordinarily  assume  the  forms  under 
which  they  are  known  as  cells ;  and  considering  as  aggregates 
of  the  second  order,  those  assemblages  of  such  cells  which, 
in  the  lower  cryptogamia,  compose  the  various  kinds  of  thal- 
lus ;  then  that  structure,  common  to  the  higher  cryptogams 
and  to  phaBnogams,  in  which  we  find  a  series  of  such  groups 

relatively  more  subordinate  to  the  forces  exerted  on  them  by  the  larger  aggre- 
gates of  molecules  that  are  at  greater  distances,  and  thus  are  left  to  arrange 
themselves  round  fewer  axes  into  larger  crystals. 


THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS.  lO 

of  cells  bound  up  into  a  continuous  whole,  must  be  regarded 
as  an  aggregate  of  the  third  order.  The  inference  drawn 
i'rom  analysis,  and  verified  by  a  synthesis  that  corresponds  in 
a  remarkable  manner  with  the  facts,  is,  that  those  compound 
parts  which,  in  Endogens  and  Exogens,  are  called  axes, 
have  really  arisen  by  integration  of  such  simple  parts  as  in 
lower  plants  are  called  fronds.  Here,  on  a  higher  level,  ap- 
pears to  have  taken  place  a  repetition  of  the  process  already 
observed  on  lower  levels.  The  formation  of  those  small 
groups  of  physiological  units  which  compose  the  lowest 
protophytes,  is  itself  a  process  of  integration  ;  and  the  con- 
solidation of  such  groups  into  definitely-circumscribed  and 
coherent  cells,  is  a  completing  of  the  process.  In  those 
coalescences,  variously  carried  on,  by  which  many  such  cells 
are  joined  into  threads,  and  discs,  and  solid  or  flattened- 
out  masses,  we  see  these  morphological  units  aggregating 
into  units  of  a  compound  kind — the  different  phases  of  the 
transition  being  exemplified  by  groups  of  various  sizes, 
various  degrees  of  cohesion,  and  various  degrees  of  definite- 
ness.  Once  more  do  we  now  find  evidences  of  a  like  process 
on  a  larger  scale :  the  compound  groups  are  again  com- 
pounded. And,  as  before,  there  are  not  wanting  types  of 
organization  by  which  the  stages  of  this  higher  integration 
are  shadowed  forth.  From  fronds  that  occasionally  produce 
ether  fronds  from  their  surfaces,  we  pass  to  those  that 
habitually  produce  them.  From  those  that  do  so  in  an  in- 
definite manner,  to  those  that  do  so  in  a  definite  manner. 
And  from  those  that  do  so  singly,  to  those  that  do  so  doubly 
and  triply  through  successive  generations  of  fronds.  Even 
within  the  limits  of  a  sub-class,  we  find  gradations  between 
fronds  irregularly  proliferous,  and  groups  of  such  fronds 
united  into  a  regular  series. 

Nor  does  the  process  end  here.  The  flowering  plant  is 
rarely  uniaxial — it  is  nearly  always  multiaxial.  From  its 
primary  shoot,  there  grow  out  secondary  shoots  of  like  kind. 
Though  occasionally  among  Phaenogams,  and  frequently 


fQ  MOUrHOLOGICAL    DEVELOPMENT. 

among  the  higher  Cryptogams,  the  germs  of  new  axes  detach 
themselves  under  the  form  of  bulbils,  and  develop  separately 
instead  of  in  connexion  with  the  parent  axis ;  yet  in  most 
PhoDiiogams,  the  germ  of  each  new  axis  maintains  its  con- 
nexion with  the  parent  axis :  whence  results  a  group  of  axes 
— an  aggregate  of  the  fourth  order.  Every  tree,  by  the  pro- 
duction of  branch  out  of  branch,  shows  us  this  integration 
repeated  over  and  over  again :  forming  an  aggregate  having 
a  degree  of  composition  too  complex  to  be  any  longer  defined. 


CHAPTER  IT. 

THE  MORPHOLOGICAL   COMPOSITION   OF   ANIMALS. 

§  199.  WHAT  was  said  in  §  180,  respecting  the  ultimate 
structure  of  organisms,  holds  more  manifestly  of  animals 
than  of  plants.  That  throughout  the  vegetal  kingdom  the 
cell  is  the  morphological  unit,  is  a  proposition  admitting  of  a 
better  defence,  than  the  proposition  that  the  cell  is  the  mor- 
phological unit  throughout  the  animal  kingdom.  The  qualifi- 
cations with  which,  as  we  saw,  the  cell-doctrine  must  be 
taken,  are  qualifications  thrust  upon  us  more  especially  by 
the  facts  which  zoologists  have  brought  to  light.  It  is 
among  the  Protozoa  that  there  occur  numerous  cases  of  vital 
activity  displayed  by  specks  of  protoplasm ;  and  from  the 
minute  anatomy  of  all  creatures  above  these,  up  to  the  Teleozoa, 
are  drawn  the  numerous  proofs  that  non-cellular  tissues  may 
arise  by  direct  metamorphosis  of  structureless  colloidal  sub- 
stance. 

Our  survey  of  morphological  composition  throughout  the 
animal  kingdom,  must  therefore  begin  with  those  undiffer- 
entiated  aggregates  of  physiological  units,  out  of  which  are 
formed  what  we  call,  with  considerable  license,  morphological 
units. 

§  200.  In  that  division  of  the  Protozoa  distinguished  as 
Rhizopoda,  are  presented,  under  various  modifications,  these 
minute  portions  of  living  organic  matter,  so  little  different!- 


78  MOIU'HOLOGICAL    DEVELOPMENT. 

ated,  if  not  positively  undifferentiated,  that  animal  individu- 
ality can  scarcely  be  claimed  for  them.     Figs.  131,  132,  and 


133,  represent  certain  nearly-allied  types  of  these — Amoebi, 
ActinopJirys,  and  Lieberkuhnia.  The  viscid  jelly  or  sarcode, 
comparable  in  its  physical  properties  to  white  of  egg,  out  of 
which  one  of  these  creatures  is  mainly  formed,  shoAVS  us  in 
various  ways,  the  feebleness  with  which  the  component  physio- 
logical units  are  integrated — shows  us  this  by  its  very  slight 
cohesion,  by  the  extreme  indefiniteness  and  mutability  of  its 
form,  and  by  the  absence  of  a  limiting  membrane.  Though 
unqualified  adherents  of  the  cell-doctrine  assert  that  the 
Amoeba  has  "an  investment,  yet  since  this  investment,  com- 
pared by  Dujardin  to  the  film  which  forms  on  the  surface  of 
paste,  does  not  prevent  the  taking  of  solid  particles  into  the 
mass  of  the  body,  and  does  not,  in  such  kindred  forms  as  Fig. 
133,  prevent  the  pseudopodia  from  coalescing  when  they 
meet,  it  cannot  be  anything  deserving  the  name  of  a  cell- 
wall.  A  considerable  portion  of  the  body,  however,  in  Difflu- 
cjia,  Fig.  134,  has  a  denser  coating ;  so  that  the  protrusion  of 
the  pseudopodia  is  limited  to  one  part  of  it.  And  in  the 
solitary  Foraminifera,  like  Gromia,  the  sarcode  is  covered 
over  most  of  its  surface  by  a  delicate  calcareous  shell,  pierced 
with  minute  holes,  through  which  the  slender  pseudopodia 
are  thrust.  The  Gregarina  exhibits  an  advance  in 

integration,  and  a  consequent  greater  definiteness.  Figs. 
135  and  136,  exemplifying  this  type,  show  the  complete 
membrane  in  which  the  substance  of  the  creature  is  con- 
tained. Here  there  has  arisen  what  may  be  properly  called 
a  cell :  under  its  solitary  form  this  animal  is  truly  unicellular. 
Its  embryology  has  considerable  significance.  After  passing 
through  a  certain  quiescent,  "  encysted "  state,  its  interior 
breaks  up  into  small  portions,  which,  after  their  exit,  assume 


THE    MORPHOLOGICAL    COMPOSITION    OF    ANIMALS.  79 

forms  like  that  of  the  Amoeba ;  and  from  this  young  condi- 
tion in  which  they  are  undifferentiated,  they  pass  into  that 
adult  condition  in  which  they  have  limiting  membranes.  If 
this  development  of  the  individual  Greyarina  typifies  the 
mode  of  evolution  of  the  species,  it  yields  further  support  to 
the  belief,  that  homogeneous  fragments  of  sarcode  existed 
earlier  than  any  of  the  structures  which  are  properly  called 
cells.  Among  aggregates  of  the  first  order,  there 

are  some  much  more  highly  dcAreloped.  These  are  the  Infu- 
soria ;  constituting  the  most  numerous  of  the  Protozoa,  in 
species  as  in  individuals.  Figs.  137,  138,  and  139,  are  ex- 
amples. In  them  we  find,  along  with  greater  defmiteness, 
a  considerable  heterogeneity.  The  sarcode  of  which  the  bod} 
consists,  has  an  indurated  outer  layer,  bearing  cilia  and  some- 
times spines ;  there  is  an  opening  serving  as  mouth,  a  per- 
manent oesophagus,  and  a  cavity  or  cavities,  temporarily 
formed  in  the  interior  of  the  sarcode,  to  serve  as  one  or  more 
stomachs  ;  and  there  is  a  comparatively  specific  arrangement 
of  these  and  various  minor  parts. 

Thus  in  the  animal  kingdom,  as  in  the  vegetal  kingdom, 
there  exists  a  class  of  minute  forms  having  this  peculiarity, 
that  no  one  of  them  is  separable  into  a  number  of  visible  com- 
ponents homologous  with  one  another — no  one  of  them  can 
be  resolved  into  minor  individualities.  Its  proximate  units 
are  those  physiological  units  of  which  we  conclude  every  or- 
ganism consists.  The  aggregate  is  an  aggregate  of  the  first 
order. 

§  201.  Among  plants  are  found  types  indicating  a  transi- 
tion from  aggregates  of  the  first  order  to  aggregates  of  the 
second  order ;  and  among  animals  we  find  analogous  types. 
But  the  stages  of  progressing  integration  are  not  here  so  dis- 
tinct. The  reason  probably  is,  that  the  simplest  animals, 
having  individualities  much  less  marked  than  those  of  the 
simplest  plants,  do  not  afford  us  the  same  facilities  for  ob- 
servation. In  proportion  as  the  limits  of  the  minor  indi- 


80  MORPHOLOGICAL   DEVELOPMENT. 

vidualities  arc  indefinite,  the  formation  of  major  individu- 
alities out  of  them,  naturally  leaves  less  conspicuous  traces. 

Be  this  as  it  may,  however,  in  such  types  of  Protozoa  as 
the  Thalassicollte,  we  find  that  though  there  is  reason  to  re- 
gard the  aggregate  as  an  aggregate  of  the  second  order,  yet 
its  divisibility  into  minor  individualities  like  those  just  de- 
scribed, is  by  no  means  manifest.  Fig.  140,  representing 


Spkcprozoum  punctatum,  one  of  this  group,  illustrates  the  diffi- 
culty. Only  by  some  license  of  interpretation,  can  we  regard 
the  "  cellseform  bodies  "  contained  in  it,  as  the  morphological 
units  of  the  animal.  The  jelly-like  mass  in  which  they  are 
imbedded,  shows  no  signs  of  being  divisible  into  portions 
having  each  a  cell  or  nucleus  for  its  centre.*  Comparison  of 
the  various  forms  assumed  by  creatures  of  this  type,  suggests, 
contrariwise,  that  the  homogeneous  sarcode  is  primary,  and 
its  included  structures  secondary.  Among  the 

Foraminifera,  we  find  evidence  of  the  coalescence  of  aggre- 
gates of  the  first  order,  into  aggregates  of  the  second  order- 
There  are  solitary  Foraminifers,  allied  to  the  creature  repre- 
sented in  Fig.  134.  Certain  ideal  types  of  combination 

*  This  statement  seems  at  variance  with  the  figure ;  but  the  figure  is  very  in- 
accurate. Its  inaccuracy  curiously  illustrates  the  vitiation  of  evidence.  When  I 
saw  the  drawing  on  the  block,  I  pointed  out  to  the  draughtsman,  that  he  had 
made  the  surrounding  curves  much  more  obviously  related  to  the  contained  bodies, 
tlian  they  were  in  the  original  (in  Dr  Carpenter's  Foraminifera) ;  and  having 
looked  on  while  he  in  great  measure  remedied  this  defect,  thought  no  further  care 
was  needed.  Now,  however,  on  seeing  the  figure  in  the  printer's  proof,  I  find 
t'.iat  the  engraver,  swayed  by  the  same  supposition  as  the  draughtsman  that  such 
a  relation  was  meant  to  be  shown,  has  made  his  lines  represent  it  still  more  de- 
cidedly than  those  of  the  draughtsman  before  they  were  corrected.  Thus,  vague 
linear  representations,  like  vague  verbal  ones,  are  apt  to  grow  more  definite 
\\hen  repeated.  Hype. thesis  warps  perceptions  as  it  warps  thoughts. 


THE    MORPHOLOGICAL    COMPOSITION    OF    ANIMALS.  8l 

among  them,  are  shown  in  Fig.  141.  And  setting  out  from 
these,  we  may  ascend  in  various  directions  to  kinds  com- 
pounded to  an  immense  variety  of  degrees  in  an  immense 
variety  of  ways.  In  all  of  them,  however,  the  separability  of 
the  major  individuality  into  minor  individualities,  is  very  in- 
complete. The  portion  of  sarcode  contained  in  one  of  these 
calcareous  chambers,  gives  origin  to  an  external  bud ;  and 
this  presently  becomes  covered,  like  its  parent,  with  calcareous 
matter  :  the  position  in  which  each  successive  chamber  is  so 
produced,  determining  the  form  of  the  compound  shell.  But 
the  portions  of  sarcode  thus  budded  out  one  from  another,  do 
not  become  distinctly  individualized.  Fig.  142,  representing 
the  living  net- work  which  remains  when  the  shell  of  an  Or- 
bitolite  has  been  dissolved,  shows  the  continuity  that  exists 
among  the  occupants  of  its  aggregated  chambers.  Still,  the 
occupant  of  each  chamber  may  fairly  be  considered  as  homo- 
logous with  a  solitary  Foraminifer ;  and  if  so,  the  Orbitolite 
is  an  aggregate  of  the  second  order  :  this  indefinite  marking- 
ofF  of  its  morphological  units,  being  the  obverse  of  the  fact 
that  the  individualities  of  their  prototypes  are  feebly  pro- 
nounced. Forms  of  essentially  the  same  kind 
are  aggregated  in  another  manner  among  the  Spongidce. 
The  fibres  of  a  living  sponge  are  clothed  with  gelatinous 
substance,  which  is  separable  into  Amoeba-like  creatures, 
capable  of  moving  about  by  their  pseudopodia  when  detach- 
ed. These  nucleated  portions  of  sarcode,  which  are  the 
morphological  units  of  the  sponge,  lining  all  its  channels 
and  chambers,  subsist  on  the  nutritive  particles  brought  to 
them  by  the  currents  of  water  that  are  drawn  in  through 
the  superficial  pores,  and  sent  out  through  the  larger  open- 
ings— currents  produced  by  ciliated  units,  such  as  are  shown 
in  Fig.  143.  So  that,  in  the  words  of  Prof.  Huxley,  "  the 
sponge  represents  a  kind  of  subaqueous  city,  where  the  people 
are  arranged  about  the  streets  and  roads,  in  such  a  manner, 
that  each  can  easily  appropriate  his  food  from  the  water  as  it 
passes  along."  In  the  compound  Infusoria,  the 


92  MORPHOLOGICAL    DEVELOPMENT. 

component  units  remain  quite  distinct.  Being,  as  aggre- 
gates of  the  first  order,  much  more  definitely  organized, 
1  heir  union  into  aggregates  of  the  second  order  does  not  de- 
stroy their  original  individualities.  Among  the  Porticella, 
of  which  two  kinds  are  delineated  in  Figs.  144  and  145,  there 
a"re  various  illustrations  of  this :  the  members  of  the  com« 
rnunity  being  sometimes  appended  to  a  single  stem  ;  some- 
times attached  by  long  separate  stems  to  a  common  base  ;  and 
sometimes  massed  together. 

Thus  far,  these  aggregates  of  the  second  order  exhibit  but 
indefinite  individualities.  The  integration  is  physical ;  but 
not  physiological.  Though,  in  the  Tlialassicollce,  there  is  a 
shape  that  has  some  symmetry ;  and  though,  in  the  Fora- 
miniferajihe  formation  of  successive  chambers  proceeds  in  such 
methodic  ways,  as  to  produce  quite-regular  and  tolerably-spe- 
cific shells ;  yet  no  more  in  these  than  in  the  Sponges  or  the 
compound  Vorticella,  do  we  find  such  co-ordination  as  gives 
the  whole  a  life  predominating  over  the  lives  of  its  parts. 
We  have  not  yet  reached  an  aggregate  of  the  second  order, 
so  individuated  as  to  be  capable  of  serving  as  a  unit  in  still 
higher  combinations.  But  in 
the  class  Ccelenterata,  this  ad- 
vance is  displayed.  The  com- 
mon  Hydra,  habitually  taken  as 
the  type  of  the  lowest  division 
of  this  class,  has  specialized 
parts  performing  mutually-subservient  functions ;  and  thus 
exhibiting  a  total  life  distinct  from  the  lives  of  the  units. 
Fig.  146  represents  one  of  these  creatures  in  its  contracted 
state  and  in  its  expanded  state ;  while  Fig.  147  is  a 
rude  diagram  from  memory  showing  the  wall  of  this 
creature's  sack-like  body  as  seen  in  section  under  the 
microscope  :  a  and  &  being  the  outer  and  inner  cellular 
layers  ;  while  in  the  central  space  between  them,  is 
that  nucleated  substance,  or  sarcode,  or  protoplasm, 
in  which  the  cells  originate.  But  this  lowly-organized 


THE    MORPHOLOGICAL    COMPOSITION    OF    ANIMALS. 


83 


tissue  of  the  Hydra,  illustrates  a  phase  of  integration  in 
which  the  lives  of  the  minor  aggregates  are  only  par- 
tially-subordinated to  the  life  of  the  major  aggregate 
formed  by  them.  For  a  Hydra's  substance  is  separable  into 
Amoeba-like  portions,  capable  of  moving  about  independ- 
ently. Prof.  Green  quotes  Ecker,  Lewes,  and  Jager,  in  proof 
that  "  this  animal  exhibits,  at  certain  seasons  of  the  year,  u, 
tendency  to  break  up  into  particles  of  a  sarcode  aspect,  which 
retain  for  a  long  time  an  independent  vitality."  And  if  we 
bear  in  mind  how  analogous  are  the  extreme  extensibility 
and  contractility  of  a  Hydra's  body  and  tentacles,  to  the  pro- 
perties displayed  by  the  sarcode  among  Rhizopods ;  we  may 
infer  that  probably  the  movements  and  other  actions  of  a 
Hydra,  are  due  to  the  half-independent  co-operation  of  the 
Amoeba-like  individuals  composing  it. 

§  202.  A  truth  which  we  before  saw  among  plants,  we 
here  see  repeated  among  animals — the  truth  that  as  soon  as 
the  integration  of  aggregates  of  the  first  order  into  aggregates 
of  the  second  order,  produces  compound  wholes  so  specific  in 
their  shapes  and  sizes,  and  so  mutually  dependent  in  their 
parts,  as  to  have  distinct  individualities  ;  there  simultaneously 
arises  the  tendency  in  them  to  produce,  by  gemmation,  other 
such  aggregates  of  the  second  order.  The  approach  towards 
definite  limitation  in  an  organism,  is,  by  implication,  an  ap- 
proach towards  a  state  in  which  growth  passing  a  certain  point, 
results,  not  in  the  increase  of  the  old  individual,  but  in  the 
formation  of  a  new  indi- 
vidual. Thus  it  happens 
that  the  common  polype 
buds  out  other  polypes, 
some  of  which  very  shortly 
do  the  like,  as  shown  in 
Fig.  148 :  a  process  paral- 
.eled  by  the  fronds  of  sundry  Algce,  and  by  those  of  the  lower 
Jungermanniacece.  And  just  as,  among  these  last  plants,  the 


MORPHOLOGICAL    DEVELOPMENT. 


proliferously-produced  fronds,  after  growing  to  certain  sizea 
and  developing  rootlets,  detach  themselves  from  their  parent- 
fronds;  so  among  these  animals,  separation  of  the  young 
ones  from  the  bodies  of  their  parents,  ensues  when  they  have 
acquired  tolerably  complete  organizations. 

There  is  reason  to  think  that  the  parallel  holds  still  fur- 
ther. Within  the  limits  of  the  Jungermanniacece,  we  found 
that  while  some  genera  exhibit  this  discontinuous  develop- 
ment, other  genera  exhibit  a  development  that  is  similar  to 
it  in  all  essential  respects,  save  that  it  is  continuous.  And 
here  within  the  limits  of  the  Hydrozoa,  we  find,  along  with 
this  genus  in  which  the  gemmiparous  individuals  are  pre- 
sently cast  off,  other  genera  in  which  they  are  not  cast  off,  but 
form  a  permanent  aggregate  of  the  third  order.  Figs.  149 
and  150,  exemplify  these  compound  Hydrozoa — one  of  them 
showing  this  mode  of  growth  so  carried  out  as  to  produce  a 
single  axis ;  and  the  other  showing  how,  by  repetitions  of 
the  process,  lateral  axes  are  produced.  Integrations  character- 
izing certain  higher  genera  of  the  Hydrozoa,  which  swim  or 
float  instead  of  being  fixed,  are  indicated  by  Figs.  151  and 
152 :  the  first  of  them  representing  the  type  of  a  group  in 
which  the  polypes  growing  from  an 
axis,  or  coenosarc,  are  drawn  through  the 
water  by  the  rhythmical  contractions 
of  the  organs  from  which  they  hang  ; 
and  the  second  of  them  representing 
a  PhysaUa  the  component'  polypes 
of  which  are  united  into  a  cluster, 
attached  to  an  air-vessel.  It  should 
be  added  that  in  the  Rhizostomes,  the 
integration  is  carried  so  far,  that  the 
individualities  of  the  polypes  are  al- 
most lost  in  that  of  the  aggregate 
they  form. 

A   parallel   series    of    illustrations 
might  be  drawn  from  that  second  di- 


THE    MORPHOLOGICAL    COMPOSITION    OF    ANIMALS.  85 

vision  of  the  Ccelenterata,  known  as  the  Adinozoa.  Here,  too, 
we  have  a  group  of  species — the  Sea-anemonies — the  individ- 
uals of  which  are  solitary.  Here,  too,  we  have  agamogenetic 
multiplication :  occasionally  by  gemmation,  but  more  fre- 
quently by  that  modified  process  called  spontaneous  fission. 
And  here,  too,  we  have  compound  forms  resulting  from  the 
arrest  of  this  spontaneous  fission  before  it  is  complete.  To 
give  examples  is  needless  ;  since  they  would '  but  show,  in 
more  varied  ways,  the  truth  already  made  sufficiently  clear, 
that  the  compound  Coelenterata  are  aggregates  of  the  third 
order,  produced  by  integration  of  aggregates  of  the  second 
order  such  as  we  have  in  the  Hydra.  As  before,  it  is 
manifest  that  on  the  hypothesis  of  evolution,  these  higher  in- 
tegrations will  insensibly  arise,  if  the  separation  of  the  gem- 
miparous  polypes  is  longer  and  longer  postponed ;  and  that  an 
increasing  postponement  will  result  by  survival  of  the  fittest, 
if  it  profits  the  group  of  individuals  to  remain  united  instead 
of  dispersing. 

§  203.  The  like  relations  exist,  and  imply  that  the  like 
processes  have  been  gone  through,  among  those  more  highly- 
organized  animals  called  Molliiscoida.  "We  have  solitary 
individuals,  and  we  have  variously- integrated  groups  of  indi- 
viduals :  the  chief  difference  between  the  evidence  here  fur- 
nished, and  that  furnished  in  the  last  case,  being  the  absence 
of  a  type  obviously  linking  the  solitary  state  with  the  aggre- 
gated state. 

It  is  now  an  accepted  belief  that  the  creatures  named  Brachi- 
opoda,  very  abundant  in  the  so-called  palaeozoic  times,  but  at 
present  comparatively  rare,  are  akin  in  structure  to  the 
Pohjzoa ;  widely  as  they  differ  from  them  in  size.  If  we  can- 
not fairly  say  that  by  union  of  many  Brachiopods  there  would 
be  produced  a  compound  animal  like  a  Polyzoon;  yet  we  may 
fairly  say  that  were  a  small  imperfectly-developed  Brachiopod 
united  with  others  like  itself,  a  Polyzoon  would  result.  This  in- 
tegration of  aggregatesof  the  second  order,  is  carried  on  among 


86 


MORPHOLOGICAL    DEVELOPMENT. 


the  Poli/zoa  in  divers  ways,  and  with  different  degrees  of  com- 
pleteness. Thelittle  patches  of  minute  cells,  shown  as  magnified 
in  Fig.  153,  so  common  oh  the  fronds  of  sea- weeds  and  the 
surfaces  of  rocks  at  low- water  mark,  display  little  beyond  me- 
chanical combination.  The  adjacent  individuals,  though  sever- 
ally originated  by  gemmation  from  the  same  germ,  have  but 
little  physiological  dependence.  In  kindred  kinds,  however, 
as  shown  in  Figs.  154  and  155,  one  of  which  is  a  magnified 
portion  of  the  other,  the  integration  is  somewhat  greater : 
the  co-operation  of  the  united  individuals  being  shown  in 
the  production  of  those  tubular  branches  which  form  their 


common  support,  and  establish  among  them  a  more  decided 
community  of  nutrition. 

Among  the  Ascidians,  another  order  of  the  Molluscoida,  this 
general  law  of  morphological  composition  is  once  more  dis- 
played. Each  of  these  creatures  subsists  on  the  nutritive 
particles  contained  in  the  water  which  it  draws  in  through 
one  orifice  and  sends  out  through  another  ;  and  it  may  thus 
subsist  either  alone,  or  in  connexion  with  others  that  are 
in  some  cases  loosely  aggregated  and  in  other  cases  closely 
aggregated.  Fig.  156,  Phallusia  mentula,  is  one  of  the  soli- 


tary  forms.  A  type  in  which  the  individuals  are  united  by  a 
stolon  that  gives  origin  to  them  by  successive  buds,  is  shown 
in  Perr/pkora,  Fig.  157.  Among  the  BotrylUdce,  of  which  one 


THE    MORPHOLOGICAL    COMPOSITION    OF    AXIMALS.  87 

kind  is  drawn  on  a  small  scale  in  Fig.  159,  and  a  portion  of 
the  same  on  a  larger  scale  in  Fig.  158,  there  is  a  combination 
of  the  individuals  into  annular  clusters,  which  are  themselves 
imbedded  in  a  common  gelatinous  matrix.  And  in  this 
group  there  are  integrations  even  a  stage  higher,  in  which 
several  such  clusters  of  clusters  grow  from  a  single  base. 
Here  the  compounding  and  re-compounding,  appears  to 
be  carried  farther  than  anywhere  else  in  the  animal 
kingdom. 

Thus  far,  however,  among  these  aggregates  of  the  third 
order,  we  see  what  we  before  saw  among  the  simpler  aggre- 
gates of  the  second  order — we  see  that  the  component  indi- 
vidualities are  but  to  a  very  small  extent  subordinated  to  the 
individuality  made  up  of  them.  In  nearly  all  the  forms  in- 
dicated, the  mutual  dependence  of  the  united  animals  is  so 
slight,  that  they  are  more  fitly  comparable  to  societies,  of 
which  the  members  co-operate  in  securing  certain  common 
benefits.  There  is  scarcely  any  specialization  of  functions 
among  them.  Only  in  the  last  type  described  do  we  see  a 
number  of  individuals  so  completely  combined  as  to  simulate 
a  single  individual.  And  even  here,  though  there  appears  to 
be  an  intimate  community  of  nutrition,  there  is  no  physio- 
logical integration  beyond  that  implied  in  several  mouths  and 
stomachs  having  a  common  vent. 

§  204.  We  come  now  to  an  extremely  interesting  ques- 
tion. Does  there  exist  in  other  sub-kingdoms  composition  of 
the  third  degree,  analogous  to  that  which  we  have  found  so 
prevalent  among  the  Cwlenterata  and  the  Molluscoida  ?  The 
question  is  not  whether  elsewhere  there  are  tertiary  aggregates 
produced  by  the  branching  or  clustering  of  secondary  aggre- 
gates, in  ways  like  those  above  traced  ;  but  whether  elsewhere 
there  are  aggregates  which,  though  otherwise  unlike  in  the 
arrangement  of  their  parts,  nevertheless  consist  of  parts  so 
similar  to  one  another  that  we  may  suspect  them  to  be 
united  secondary  aggregates.  The  various  compound  typeiL 


88  MORPHOLOGICAL    DEVELOPMENT. 

ubove  described,  in  which  the  united  animals  maintain  their 
individualities  so  distinctly  that  the  individuality  of  the 
aggregate  remains  vague,  are  constructed  in  such  ways  that 
the  united  animals  carry  on  their  several  activities  with 
scarcely  any  mutual  hindrance.  The  members  of  a  branched 
Hydrozoon  such  as  is  shown  in  Fig.  149  or  Fig.  150,  are  so 
placed  that  they  can  all  spread  their  tentacles  and  catch 
their  prey  as  well  as  though  separately  attached  to  stones  or 
weeds.  Packed  side  by  side  on  a  flat  surface  or  forming  a  tree- 
like assemblage,  the  associated  individuals  among  the  Polyzoa 
are  not  unequally  conditioned ;  or  if  one  has  some  advantage 
over  another  in 'a  particular  case,  the  mode  of  growth  and 
the  relations  to  surrounding  objects  are  so  irregular  as  to 
prevent  this  advantage  re-appearing  with  constancy  in  suc- 
cessive generations.  Similarly  with  the  Ascidians  growing 
from  a  stolon  or  those  forming  an  annular  cluster  :  each  of 
them  is  as  well  placed  as  every  other  for  drawing  in  the 
currents  of  sea-water  from  which  it  selects  its  food.  In 
these  cases  the  mode  of  aggregation  does  not  expose  the 
united  individuals  to  multiform  circumstances ;  and  there- 
fore is  not  calculated  to  produce  among  them  any  structural 
multiformity.  For  the  same  reason  no  marked  physiologi- 
cal division  of  labour  arises  among  them ;  and  consequently 
no  combination  close  enough  to  disguise  their  several  indi- 
vidualities. But  under  converse  conditions  we  may  expect 
converse  results.  If  there  is  a  mode  of  integration  which 
necessarily  subjects  the  united  individuals  to  unlike  sets  of 
incident  forces,  and  does  this  with  complete  uniformity  from 
generation  to  generation,  it  is  to  be  inferred  that  the  united 
individuals  will  become  unlike.  They  will  severally  assume 
such  different  functions  as  their  different  positions  enable 
them  respectively  to  carry  on  with  the  greatest  advantage  to 
the  assemblage.  This  heterogeneity  of  function  arising 
among  them,  will  be  followed  by  heterogeneity  of  structure ; 
as  also  by  that  closer  combination  which  the  better  enables 
them  to  utilize  one  another's  functions.  And  hence,  while 


THE    MORPHOLOGICAL    COMPOSITION    OF    ANIMALS.  89 

the  ojiginally-like  individuals  are  rendered  unlike,  they  will 
have  tlieir  liomologies  further  obscured  by  their  progressing 
fusion  into  an  aggregate  individual  of  a  higher  order. 

These  converse  conditions  are  in  nearly  all  cases  fulfilled 
where  the  successive  individuals  arising  by  continuous  devel- 
opment are  so  budded-off  as  to  form  a  linear  series.  I  say 
in  nearly  all  cases,  because  there  are  some  types  in  which 
the  associated  individuals,  though  joined  in  single  file,  are 
not  thereby  rendered  very  unlike  in  their  relations  to  the 
environment;  and  therefore  do  not  become  differentiated  and 
intef  ;rated  to  any  considerable  extent.  I  refer  to  such  Asci- 
diam  as  the  Salpidce.  These  creatures  float  passively  in  the 
sea,  attached  together  in  strings.  Being  placed  side  by  side 
and  having  mouths  and  vents  that  open  laterally,  each  of 
them  is  as  well  circumstanced  as  its  neighbours  for  absorb- 
ing and  emitting  the  surrounding  water;  nor  have  the  in- 
dividuals at  the  two  extremities  any  marked  advantages 
over  the  rest  in  these  respects.  Hence  in  this  type,  and  in 
the  allied  type  Pi/rosoma,  which  has  its  component  indivi- 
duals built  into  a  hollow  cylinder,  linear  aggregation  may 
exist  without  the  minor  individualities  becoming  obscured 
and  the  major  individuality  marked  :  the  conditions  under 
which  a  differentiation  and  integration  of  the  component 
individuals  may  be  expected,  are  not  fulfilled.  But  where 
the  chain  of  individuals  produced  by  gemmation,  is  either 
habitually  fixed  to  some  solid  body  by  one  of  its  extremities 
or  moves  actively  through  the  water  or  over  submerged 
stones  and  weeds,  the  several  members  of  the  chain  become 
differently  conditioned  in  the  way  above  described ;  and  may 
therefore  be  expected  to  become  unlike  while  they  become 
united.  A  clear  idea  of  the  contrast  between  these  two 
linear  arrangements  and  their  two  diverse  results,  will  be 
obtained  by  considering  what  happens  to  a  row  of  soldiers, 
when  changed  from  the  ordinary  position  of  a  single  rank 
to  the  position  of  Indian  file.  So  long  as  the  men  stand 
shoulder  to  shoulder,  they  are  severally  able  to  use  their 


vO  MORPHOLOGICAL    DEVELOPMENT. 

weapons  in  like  ways  with  like  efficiency ;  and  cottld,  if 
called  on,  similarly  perform  various  manual  processes  directly 
or  indirectly  conducive  to  their  welfare.  But  when  on  the 
word  of  command  "  right  face,"  they  so  place  themselves 
that  each  has  one  of  his  neighbours  before  him  and  another 
behind  him,  nearly  all  of  them  become  incapacitated  for 
fighting  and  for  many  other  actions.  They  can  walk  or  run 
one  after  another,  so  as  to  produce  movement  of  the  file  in 
the  direction  of  its  length ;  but  if  the  file  has  to  oppose  an 
enemy  or  remove  an  obstacle  lying  in  the  line  of  its  march, 
the  front  man  is  the  only  one  able  to  use  his  weapons  or 
hands  to  much  purpose.  And  manifestly  such  an  arrange- 
ment could  become  advantageous  only  if  the  front  man  pos- 
sessed powers  peculiarly  adapted  to  his  position,  while  those 
behind  him  facilitated  his  actions  by  carrying  supplies,  &c. 
This  simile,  grotesque  as  it  seems,  serves  to  convey  better 
perhaps  than  any  other  could  do,  a  clear  idea  of  the  relations 
that  must  arise  in  a  chain  of  individuals  arising  by  gemma 
tion,  and  continuing  permanently  united  end  to  end.  Such 
a  chain  can  arise  by  natural  selection,  only  on  condition  that 
combination  is  more  advantageous  than  separation  ;  and  for 
it  to  be  more  advantageous,  the  anterior  members  of  the  series 
must  become  adapted  to  functions  facilitated  by  their  posi- 
tions, while  the  posterior  members  become  adapted  to  func- 
tions which  their  positions  permit.  Hence,  survival  of  the 
fittest  must  tend  continually  to  establish  types  in  which  the 
connected  individuals  are  more  unlike  one  another,  at  the 
same  time  that  their  several  individualities  are  more  dis- 
guised by  the  integration  consequent  on  their  mutual 
dependence. 

Such  being  the  anticipations  warranted  by  the  general 
laws  of  evolution,  we  have  now  to  inquire  whether  the*~e 
are  any  animals  which  fulfil  them.  Very  little  search 
suffices ;  for  structures  of  the  kind  to  be  expected  are  abund- 
ant. In  that  great  division  of  the  animal  kingdom  called 
Annulosa,  especially  if  the  Annuloida  be  regarded  as  part  of 


THE    MORPHOLOGICAL    COMPOSITION    OF   ANIMALS.  91 

it,  we  find  a  variety  of  types  having  the  looked- for  charac- 
ters.    Let  us  contemplate  some  of  them. 

§  205.  An  adult  Annelid  is  composed  of  segments  which 
repeat  one  another  in  their  details  as  well  as  in  their  general 
shapes.  Dissecting  one  of  the  lower  orders,  such  as  isi 
shown  in  Fig.  160,  proves  that  the  successive  segments,  be- 


Nxies  having  like  locomotive  appendages,  like  branchiae,  and 
sometimes  even  like  pairs  of  eyes,  also  have  like  internal 
organs.  Each  has  its  enlargement  of  the  alimentary  canal ; 
each  its  contractile  dilatation  of  the  great  blood-vessel ;  each 
its  portion  of  the  double  nervous  cord,  with  ganglia  when 
these  exist ;  each  its  branches  from  the  nervous  and  vascular 
trunks  answering  to  those  of  its  neighbours ;  each  its  simi- 
larly answering  set  of  muscles ;  each  its  pair  of  openings 
through  the  body- wall ;  and  so  on  throughout,  even  to  the 
organs  of  reproduction.  That  is  to  say,  every  segment  is  in 
great  measure  a  physiological  whole — every  segment  con- 
tains most  of  the  organs  essential  to  individual  life  and  mul- 
tiplication :  such  essential  organs  as  it  does  not  contain, 
being  those  which  its  position  as  one  in  the  midst  of  a  chain, 
prevents  it  from  having  or  needing.  If  we 

ask  what  is  the  meaning  of  these  homologies,  no  adequate 
answer  is  supplied  by  any  current  hypothesis.  That  this 
"  vegetative  repetition "  is  carried  out  to  fulfil  a  prede- 
termined plan,  was  shown  to  be  quite  an  untenable  notion 
(§§  133,  134).  On  the  one  hand,  we  found  nothing  satis- 
factory in  the  conception  of  a  Creator  who  prescribed  to  him- 


92  MORPHOLOGICAL   DEVELOPMENT. 

self  a  certain  unit  of  composition  for  all  creatures  of  a  par- 
ticular class,  and  then  displayed  his  ingenuity  in  building  up 
a  great  variety  of  forms  without  departing  from  the  "  arche- 
typal idea."  On  the  other  hand,  examination  made  it  mani- 
fest that  even  were  such  a  conception  worthy  of  being  enter- 
tained, it  would  have  to  be  relinquished  ;  since  in  each  class 
there  are  numerous  deviations  from  the  supposed  "  archetypal 
idea."  Still  less  can  these  traits  of  structure  be  accounted 
for  teleologically.  That  certain  organs  of  nutrition  and  re- 
spiration and  locomotion  are  repeated  in  each  segment  of  a 
dorsibranchiate  annelid,  may  be  regarded  as  functionally  ad- 
vantageous for  a  creature  following-  its  mode  of  life.  But 
why  should  there  be  a  hundred  or  even  two  hundred  pairs  of 
ovaries?  This  is  an  arrangement  at  variance  with  that 
physiological  division  of  labour  which  every  organism  pro- 
fits by — is  a  less  advantageous  arrangement  than  might  have 
been  adopted.  That  is  to  say,  the  hypothesis  of  a  designed 
adaptation  fails  to  explain  the  facts.  Contrariwise, 

these  structural  traits  are  just  such  as  might  naturally  be 
looked  for,  if  these  annulose  forms  have  arisen  by  the  in- 
tegration of  simpler  forms.  Among  the  various  compound 
animals  already  glanced  at,  it  is  very  general  for  the  united 
individuals  to  repeat  one  another  in  all  their  parts — repro- 
ductive organs  included.  Hence  if,  instead  of  a  clustered  or 
branched  integration,  such  as  the  Caclentcrata  and  Mollmcoida 
exhibit,  there  occurs  a  longitudinal  integration ;  we  may  ex- 
pect that  the  united  individuals  will  habitually  indicate  their 
original  independence  by  severally  bearing  germ-producing 
or  sperm-producing  organs. 

The  reasons  for  believing  one  of  these  creatures  to  be  an 
aggregate  of  the  third  order,  are  greatly  strengthened  when 
we  turn  from  the  adult  structure  to  the  mode  of  develop- 
ment. Among  the  Dorsibranchiata  and  Tiibicola,  the  em- 
bryo leaves  the  egg  in  the  shape  of  a  ciliated  gemmule,  not 
much  more  differentiated  than  that  of  a  polype.  As  shown 
in  Fig.  162,  it  is  a  nearly  globular  mass ;  and  its  interior 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  93 

consists  of  untransformed  cells.  The  first  appreciable  change 
is  an  elongation  and  a  simultaneous  commencement  of  seg- 
mentation. The  segments  multiply  by  a  modified  gemma- 
tion, which  takes  place  from  the  hinder  end  of  the  penultimate 
segment.  And  considerable  progress  in  marking  out  these 
divisions  is  made  before  the  internal  organization  begins. 
Figs.  163, 164, 165,  represent  some  of  these  early  stages.  In 


set 

/63 


S65 

Annelids  of  other  orders,  the  embryo  assumes  the  segmented 
form  while  still  in  the  egg.  But  it  does  this  in  just  the 
same  manner  as  before.  Indeed,  the  essential  identity  of  the 
two  modes  of  development  is  shown  by  the  fact  that  the  seg- 
mentation within  the  egg  is  only  partially  carried  out :  in" 
all  these  types  the  segments  continue  to  increase  in  number 
for  some  time  after  birth.  Now  this  process  is  as 

like  that  by  which  compound  animals  in  general  are  formed, 
as  the  different  conditions  of  the  case  permit.  When  new 
individuals  are  budded-out  laterally,  their  unfolding  is  not 
hindered — there  is  nothing  to  disguise  either  the  process  or 
the  product.  But  gemmae  produced  one  from  another  in  the 
same  straight  line,  and  remaining  connected,  restrict  one 
another's  developments ;  and  that  the  resulting  segments  are 
so  many  gemmiparously-produced  individuals,  is  necessarily 
less  obvious. 

§  206.  Evidence  remains  which  adds  very  greatly  to  the 
weight  of  that  already  assigned.  Thus  far  we  have  studied 
only  the  individual  annulose  animal ;  considering  what  may 
be  inferred  from  its  mode  of  evolution  and  final  organization. 


94  MORPHOLOGICAL    DEVELOPMENT. 

We  have  now  to  study  annulose  animals  in  general.  Com- 
parison of  them  will  disclose  various  phases  of  progiessive 
integration  of  the  kind  to  be  anticipated. 

Among  the  simpler  Annuloida,  as  in  the  Nemcrtidcr,  and  in 
some  kinds  of  Planaria,  transverse  fission  occurs.  A  por- 
tion of  a  Planaria  separated  by  spontaneous  constriction, 
becomes  an  independent  individual.  Sir  J.  G.  Dalyell  found 
that  in  some  cases  numerous  fragments  artificially  separated, 
grew  into  perfect  animals.  In  these  annuloids  which  thus 
remind  us  of  the  lowest  Hydrozoa  in  their  powers  of  agamo- 
genetic  multiplication,  the  individuals  produced  one  from 
another,  do  not  continue  connected.  As  the  young  ones 
laterally  budded-off  by  the  Hydra  separate  when  complete, 
so  do  the  young  ones  longitudinally  budded-off  by  the  Pla- 
naria. Fig.  166  indicates  this.  But  there  are  allied  types 
which  show  us  a  more  or  less  persistent  union  of  homologous 
parts,  or  individuals,  similarly  arising  by  longitudinal  gem- 
ination. The  cestoid  Entozoa  furnish  illustrations.  Without 
dwelling  on  the  fact  that  each  segment  of  a  Tcenia,  like  each 
separate  Planaria,  is  an  independent  hermaphrodite,  or  on  the 
fact  that  both  develop  their  ova  by  the  peculiar  method  of 
forming  germinal  vesicles  in  one  canal  and  surrounding  them 
with  yelk  that  is  secreted  in  another  canal ;  and  without 
specifying  the  sundry  common  structural  traits  which  add 
probability  to  the  suspicion  that  there  is  some  kinship  be- 
twesn  the  individuals  of  the  one  order  and  the  segments  of 
the  other ;  it  will  suffice  to  point  out  that  the  two  types  are 
so  far  allied  as  to  demand  their  union  under  the  same  sub- 
class title.  And  recognizing  this  kinship,  we  see  significance 
in  the  fact  that  in  the  one  case  the  longitudinally-produced 
gemmae  separate  as  complete  individuals,  and  in  the  other 
continue  united  as  segments  in  smaller  or  larger  numbers 
and  for  shorter  or  longer  periods.  In  Tcenia  echinococcits, 
represented  in  Fig.  167,  we  have  a  species  in  which  the 
number  of  segments  thus  united  does  not  exceed  four.  In 
Echinobothrium  typns  there  Jire  eight  or  ten  ;  and  in  cestoids 


THE    MORPHOLOGICAL    COMPOSITION    OF    ANIMALS. 


generally  they  are  numerous.*  A  considerable 

hiatus  occurs  between  this  phase  of  integration  and  the  next 
higher  phase  which  we  meet  with;  but  it  is  not  greater 
than  the  hiatus  between  the  types  of  the  Annuloida  and  the 
Annelida,  which  present  the  two  phases.  Though  it  is 
doubtful  whether  separation  of  single  segments  occurs  among 
the  Annelida,  yet  very  often  we  find  strings  of  segments, 
arising  by  repeated  longitudinal  budding,  which  after  reach- 
ing certain  lengths  undergo  spontaneous  fission  :  in  some 
cases  doing  this  so  as  to  form  two  or  more  similar  strings 
of  segments  constituting  independent  individuals  ;  and  in 
other  cases  doing  it  so  that  the  segments  spontaneously 
separated  are  but  a  small  part  of  the  string.  Thus  a  Syllix, 
Fig.  168,  after  reaching  a  certain  length,  begins  to  trans- 


0 

166 


form  itself  into  two  individuals  :  one  of  the  posterior  seg- 
ments develops  into  a  head,  and  simultaneously  narrows  its 
connexion  with  the  preceding  segments,  from  which  it 

*  I  find  that  the  reasons  for  regarding  the  segment  of  a  Tcenia  as  answering 
to  an  individual  of  the  second  order  of  aggregation,  are  much  stronger  than  I  sup- 
posed when  writing  the  above.  Van  Beneden  says : — "  Le  Proglottis  (segment) 
ayant  acquis  tout  son  developpement,  se  detache  ordinairement  de  la  colonie  et 
continue  encore  a  croitre  dans  1'intestin  du  mcme  animal ;  il  change  meme  sou- 
vent  de  form  ft  et  semble  doue  d'une  nouvelle  vie ;  ses  angles  s'effacent,  tout  le  corps 
s'arron.lit,  et  il  na^e  comme  une  Planaire  au  milieu  des  -nuscosites  intestinales." 
VOL.  II.  5 


96  MORPHOLOGICAL   DEVELOPMENT. 

eventually  separates.  Still  more  remarkable  is  tlie  extent  to 
which  this  process  is  carried  in  certain  kindred  types ;  which 
exhibit  to  us  several  individuals  thus  being  simultaneously 
formed  out  of  groups  of  segments.  Fig.  169,  copied  (omit- 
ting the  appendages)  from  one  contained  in  a  memoir 
by  M.  Milne-Edwards,  represents  six  worms  of  different 
ages  in  course  of  development :  the  terminal  one  being  the 
eldest,  the  one  having  the  greatest  number  of  segments, 
and  the  one  that  will  first  detach  itself;  and  the  success- 
ively anterior  ones,  with  their  successively  smaller  numbers 
of  segments,  being  successively  less  advanced  towards  fitness 
for  separation  and  independence.  Here  among  groups  of 
segments  we  see  repeated  what  in  the  previous  cases  occurs 
with  single  segments.  And  then  in  other  Annelids  we  find  that 
the  string  of  segments  arising  by  gemmation  from  a  single 
germ  becomes  a  permanently  united  whole  :  the  tendency  to 
any  more  complete  fission  than  that  which  marks  out  the  seg- 
ments, being  lost ;  or,  in  other  words,  the  integration  having 
become  relatively  complete.  Leaving  out  of  sight  the 

question  of  alliance  among  the  types  above  grouped  together, 
that  which  it  here  concerns  us  to  notice  is,  that  longitudinal 
gemmation  does  go  on  ;  that  it  is  displayed  in  that  primitive 
form  in  which  the  gemmae  separate  as  soon  as  produced ;  that 
we  have  types  in  which  such  gemmae  hang  together  in 
groups  of  four,  or  in  groups  of  eight  and  ten,  from  which 
however  the  gemmae  successively  separate  as  individuals ; 
that  among  higher  types  we  have  long  strings  of  similarly- 
formed  gemmae  which  do  not  become  individually  independ- 
ent, but  separate  into  organized  groups ;  and  that  from 
these  we  advance  to  forms  in  which  all  the  gemmae  remain 
parts  of  a  single  individual.  One  other  significant 

olass  of  facts  must  be  added.  A  few  cases  have  been  pointed 
out,  one  of  them  quite  recently,  in  which  Annelids  mul- 
tiply by  lateral  gemmation.  M.  Pagenstecher  alleges  this 
of  the  Exogone  gemmifem :  describing  a  certain  number  of 
the  segments  of  the  body  as  severally  bearing  on  their  dorsal 


THE   MORPHOLOGICAL   COMPOSITION   OF    ANIMALS.  97 

surfaces  a  bud  on  each  side.  And  M.  L.  Vaillant,  after 
citing  this  observation  of  M.  Pageiistecher,  gives  an  account 
of  a  species  of  Si/ His  in  which  a  great  number  of  buds  were 
borne  by  a  single  segment.  That  the  longitudinally-produced 
gemmoe  which  compose  an  Annelid,  should  thus  have,  one  of 
them  or  several  of  them,  the  power  of  laterally  budding-off 
gemmae,  from  which  no  doubt  other  annelids  arise,  gives  fur- 
ther support  to  the  hypothesis  that,  primordially,  the  seg- 
ments were  independent  individuals.  And  it  suggests  this  be- 
lief the  more  strongly  because,  in  certain  types  of  Coelentcrata, 
we  see  that  longitudinal  and  lateral  gemmation  do  occur  to- 
gether, where  the  longitudinally-united  gemmae  are  demon- 
strably  independent  individuals. 

§  207.  It  would  add  to  the  probability  of  this  conclusion 
could  we  identify  the  type  out  of  which  the  annulose  type 
may  have  arisen  by  the  process  of  integration.  I  believe 
there  may  be  pointed  out  such  a  type — a  type  which,  by  a 
slight  modification  carrying  somewhat  further  an  habitual 
mode  of  development,  would  produce  not  only  a  unit  of  com- 
position for  the  annulose  type,  but  also  as  a  bond  uniting  it 
with  the  other  types,  and  these  with  one  another.  It  is  un- 
desirable, however,  here  to  enter  upon  the  numerous  explan- 
ations involved  by  opening  the  question  of  these  relation- 
ships ;  both  because  it  would  necessitate  a  long  digression, 
encumbering  too  much  the  general  argument,  and  because, 
being  highly  speculative,  it  would  be  impolitic  to  let  tb.3 
general  argument  be  even  apparently  implicated  by  it. 

But  even  supposing  it  impossible  now  to  identify  the  unit  of 
composition  of  the  annulose  type,  the  foregoing  evidence  still 
goes  far  towards  showing  that  an  annulose  animal  is  an  aggre- 
gate of  the  third  order.  This  repetition  of  segments,  some- 
times numbering  several  hundreds,  like  one  another  in  all 
their  organs  even  down  to  those  of  reproduction,  while  it  is 
otherwise  unaccountable,  is  fully  accounted  for  if  these  seg- 
ments are  homologous  with  the  separate  individuals  of  some 


98  MORPHOLOGICAL    DEVELOPMENT. 

lower  type.  The  gemmation  by  which  these  segments  are  pro- 
duced, is  as  similar  as  the  conditions  allow,  to  the  gemmation 
by  which  compound  animals  in  general  are  produced.  As 
among  plants  and  as  among  demonstrably-compound  animals, 
we  see  that  the  only  thing  required  for  the  formation  of  a  per- 
manent chain  of  gemmiparously-produced  individuals,  is  that 
by  remaining  associated,  such  individuals  will  have  advantages 
greater  than  are  to  be  gained  by  separation.  Further,  by 
comparison  of  the  annuloid  and  lower  annulose  forms,  we 
discover  a  number  of  those  transitional  phases  of  integration 
which  the  hypothesis  leads  us  to  expect.  And,  lastly,  the 
differences  among  these  united  individuals  or  successive 
segments,  are  not  greater  than  the  differences  in  their  posi- 
tions and  functions  explain — not  greater  than  such  differences 
are  known  to  produce  among  other  united  individuals  :  wit- 
ness sundry  compound  Hydrozoa. 

Indirect  evidence  of  much  weight  has  still  to  be  given. 
Thus  far  we  have  considered  only  the  less-developed  Annn- 
losa.  The  more  integrated  and  more  differentiated  types  of 
the  class  remain.  If  in  them  we  find  a  carrying  further  of 
the  processes  by  which  the  lower  types  are  here  supposed  to 
have  been  evolved,  we  shall  have  additional  reason  for  be- 
lieving them  to  have  been  so  evolved.  If  we  find  that  in 
these  superior  orders  the  individualities  of  the  united  seg- 
ments are  much  less  pronounced  than  in  the  inferior,  we 
shall  have  grounds  for  suspecting  that  in  the  inferior  the 
individualities  of  the  segments  are  less  pronounced  than  in 
those  lost  forms  which  initiated  the  annulose  sub-kingdom. 


CHAPTER  V. 

THE   MORPHOLOGICAL   COMPOSITION    OF   ANIMALS, 
CONTINUED. 

§  208.  INSECTS,  Arachnids,  Crustaceans,  and  Myriapods, 
are  all  members  of  that  higher  division  of  the  Annulosa  called 
Articulata  or  Arthropoda.  Though  in  these  creatures  the 
formation  of  segments  may  be  interpreted  as  a  disguised 
gemmation  ;  and  though  in  some  of  them  the  number  of  seg- 
ments increases  by  this  modified  budding  after  leaving  the 
egg,  as  among  the  higher  Annelids ;  yet  the  process  is  not 
nearly  so  dominant :  the  segments  are  usually  much  less 
numerous  than  we  find  them  in  the  types  last  considered. 
In  most  cases,  too,  the  segments  are  in  a  greater  degree  dif- 
ferentiated one  from  another,  at  the  same  time  that  they 
are  severally  more  differentiated  within  themselves.  Nor  is 
there  any  instance  of  spontaneous  fission  taking  place  in  the 
series  of  segments  composing  an  articulate  animal.  On  the 
contrary,  the  integration,  always  great  enough  permanently 
to  unite  the  segments,  is  frequently  carried  so  far  as  to  hide 
very  completely  the  individualities  of  some  or  many  of  them ; 
and  occasionally,  as  among  the  Acari,  the  consolidation,  or 
the  arrest  of  segmentation,  is  so  decided  as  to  leave  scarcely 
a  trace  of  the  articulate  structure  :  the  type  being  in  these 
cases  indicated  chiefly  by  the  presence  of  those  character- 
istically-formed limbs,  which  give  the  alternative  name 
Arthropoda  to  all  the  higher  Annulosa.  Omitting  the  para- 
sitic orders,  which,  as  in  other  cases,  are  aberrant  members  of 


100 


MORPHOLOGICAL    DEVELOPMENT. 


their  sub-kingdom,  comparisons  between  the  different  orders 
prove  that  the  higher  are  strongly  distinguished  from  the 
lower,  by  the  much  greater  degree  in  which  the  individual- 
ity of  the  tertiary  aggregate  dominates  over  the  individual- 
ities of  those  secondary  aggregates  called  segments  or 
"  somites,"  of  which  it  is  composed.  The  successive  Figs. 
170 — 176,  representing  (without  their  limbs)  a  Julus,  a 


Scolopendra,  an  isopodous  Crustacean,  and  four  kinds  of 
decapodous  Crustaceans,  ending  with  a  Crab,  will  convey  at  a 
glance  an  idea  of  the  way  in  which  that  greater  size  and 
heterogeneity  reached  by  the  higher  types,  is  accompanied 
by  an  integration  which,  in  the  extreme  cases,  almost  obliter- 
ates all  traces  of  composite  structure.  In  the  Crab  the 
posterior  segments,  usually  folded  underneath  the  shell, 
alone  preserve  their  primitive  distinctness :  so  completely 
confluent  are  the  rest,  that  it  seems  absurd  to  say  that  a 
Crab's  carapace  is  composed  of  as  many  segments  as  there  are 
pairs  of  limbs,  foot-jaws,  and  antennae  attached  to  it ;  and 
were  it  not  that  during  early  stages  of  the  Crab's  develop- 
ment the  segmentation  is  faintly  marked,  the  assertion  might 
be  considered  illegitimate. 

That  all  articulate  animals  are  thus  composed  from  end  to 
end  of  homologous  segments,  is,  however,  an  accepted  doc- 
trine among  naturalists.  It  is  a  doctrine  that  rests  on  care- 


THE   MORPHOLOGICAL    COMPOSITION   OF   ANIMALS.        101 

ful  observation  of  three  classes  of  facts — the  correspondences 
of  parts  in  the  successive  '•  somites  "  of  an  adult  articulate 
animal ;  the  still  more  marked  correspondences  of  such 
parts  as  they  exist  in  the  embryonic  or  larval  articulate  ani- 
mal ;  and  the  maintenance  of  such  correspondences  in  some 
types,  which  are  absent  in  types  otherwise  near  akin  to  them. 
The  nature  of  the  conclusion  which  these  evidences  unite  in 
supporting,  will  best  be  shown  by  the  annexed  copies  from 
the  lecture -diagrams  of  Prof.  Huxley;  exhibiting  the 
typical  structures  of  a  Myriapod,  an  Insect,  a  Spider,  and  a 
Crustacean,  with  their  relations  to  a  common  plan,  as  in- 
terpreted by  him. 


Head          Thorax  Abdomen 


/S7 


Treating  of  these  homologies,  Prof.  Huxley  says  "  that  a 
etriking  uniformity  of  composition  is  to  be  found  in  the  heads 
of,  at  any  rate,  the  more  highly  organized  members  of  these 
four  classes,  and  that,  typically,  the  head  of  a  Crustacean, 
an  Arachnid,  a  Myriapod,  or  an  Insect,  is  composed  of  six 
somites  (or  segments  corresponding  with  those  of  the  body) 
and  their  appendages,  the  latter  being  modified  so  as  to 
serve  the  purpose  of  sensory  and  manducatory  organs." 
And  omitting  the  Myriapods,  he  also  finds  among  these 
groups  the  further  unity  that  in  most  of  them  the  entire 
animal  contains  twenty  of  these  homologous  segments. 


102  MORPHOLOGICAL   DEVELOPMENT. 

Thus  even  in  the  higher  Annulosa,  the  much  greater  conso. 
lidation  and  much  greater  heterogeneity  do  not  obliterate  the 
evidence  of  the  fact,  that  the  organism  is  an  aggregate  of 
the  third  order.  Beyond  all  question  it  is  divisible  into  a 
number  of  proximate  units,  each  of  which  has  essentially  the 
same  structure  as  its  neighbours,  and  each  of  which  is  an 
aggregate  of  the  second  order,  in  so  far  as  it  is  an  organized 
combination  of  those  aggregates  of  the  first  order  which  we 
call  morphological  units  or  cells.  And  that  these  segments 
or  somitea,  which  make  up  an  annulose  animal,  were  origin- 
ally aggregates  of  the  second  order  having  independent  in- 
dividualities, is  an  hypothesis  which  gathers  further  support 
from  the  contrast  between  the  higher  and  the  lower  articu- 
late types,  as  well  as  from  the  contrast  between  the  Artlcu- 
lata  in  general  and  the  inferior  Annulosa.  For  if  that 
masking  of  the  individualities  of  the  segments  which  we  find 
distinguishes  the  higher  forms  from  the  lower,  has  been  going 
on  from  the  beginning,  as  we  may  fairly  assume  ;  it  is  to  be 
inferred  that  the  individualities  of  the  segments  in  the  lower 
forms,  were  originally  more  marked  than  they  now  are. 
Reversing  those  processes  of  change  by  which  the  most 
developed  Annulosa  have  arisen  from  the  least  developed ; 
and  applying  in  thought  this  reversed  process  to  the  least 
developed,  as  they  were  described  in  the  last  Chapter  ;  we 
are  brought  to  the  conception  of  attached  segments  that  are 
completely  alike,  and  have  their  individualities  in  no  ap- 
preciable degree  subordinated  to  that  of  the  chain  they  com- 
pose. From  which  there  is  but  a  step  to  the  conception  of 
gemmiparously-produced  individuals  which  severally  part 
one  from  another  as  soon  as  they  are  formed. 

§  209.  "We  must  now  return  to  a  point  whence  we  di- 
verged some  time  ago.  As  before  explained  under  the  head 
of  Classification,  organisms  do  not  admit  of  uni-serial  ar- 
rangement, either  in  general  or  in  detail ;  but  everywhere 
form  groups  within  groups.  Hence,  having  traced  the 


THE    MORPHOLOGICAL   COMPOSITION    OF   ANIMALS.         103 

phases  of  morphological  composition  up  to  the  highest  forms 
in  any  sub-kingdom,  we  find  ourselves  at  the  extremity  of  a 
great  branch,  from  which  there  is  no  access  to  another  great 
branch,  except  by  going  back  to  some  place  of  bifurcation  low 
down  in  the  tree. 

The  nearest  relatives  of  the  Mollusca  are  those  molluscoid 
forms  treated  of  early  in  the  last  Chapter.  A  Brachiopod  or 
a  solitary  Ascidian,  though  widely  unlike  a  Mussel  or  a 
Snail  or  a  Cuttle-fish,  is  nearer  akin  to  them  than  is  any 
ccelenterate  animal  or  annulose  animal  or  vertebrate  animal. 
One  of  the  leading  distinctions,  however,  between  the  Mol- 
luscoida  and  the  Mollusca,  considered  as  groups,  is  that 
whereas  the  Molluscoida  are  very  frequently,  or  indeed 
generally,  compound,  the  Mollusca  are  invariably  single. 
No  true  Mollusk  multiplies  by  gemmation,  either  continuous 
or  discontinuous  ;  but  the  product  of  every  fertilized  germ  is 
a  single  individual. 

It  is  a  significant  fact  that  here,  where  for  the  first  time 
we  have  homogenesis  holding  throughout  an  entire  sub- 
kingdom,  we  have  also  throughout  an  entire  sub-king- 
dom no  case  in  which  the  organism  is  divisible  into  two, 
three,  or  more,  like  parts.  There  is  neither  any  such 
clustering  or  branching  as  a  ccelenterate  or  molluscoid  ani- 
mal usually  displays  ;  nor  is  there  any  trace  of  that  seg- 
mentation which  characterizes  the  Annulosa.  Among  these 
animals  in  which  no  single  egg  produces  several  individuals, 
no  individual  is  separable  into  several  homologous  divisions. 
This  connexion  will  be  seen  to  have  a  probable  meaning,  on 
remembering  that  it  is  the  converse  of  the  connexion  which 
obtains  among  the  Annulosa,  considered  as  a  group. 

A  Mollusk,  then,  is  an  aggregate  of  the  second  order.  Not 
only  in  the  adult  animal  is  there  no  sign  of  a  multiplicity  of 
like  parts  that  have  become  obscured  by  integration ;  but 
there  is  no  sign  of  such  multiplicity  in  the  embryo.  And 
this  unity  is  just  as  conspicuous  in  the  lowest  Lamelli- 
branch  as  in  the  highest  Cephalopod. 


[04  .          MOKPIIOLOGICAL   DEVELOPMENT. 

It  may  be  well  to  note,  however,  more  especially  because 
it  illustrates  a  danger  of  misinterpretation  presently  to  be 
guarded  against,  that  there  are  certain  Mollusks  which  si- 
mulate the  segmented  structure.  Externally  a  Chiton,  Fig. 
188,  appears  to  be  made  up 
of  divisions  substantially  like 
those  of  the  creature  Fig. 
189 ;  and  one  who  judged 
only  by  externals,  would  say 
that  the  creature  Fig.  190 
differs  as  much  from  the 
creature  Fig.  189,  as  this 
does  from  the  preceding  one.  But  the  truth  is,  that  while 
190  and  189  are  closely-allied  types,  189  differs  from  188 
much  more  widely  than  a  man  does  from  a  fish.  And  the 
radical  distinction  between  them  is  this ;  that  whereas  in  the 
Crustacean  the  segmentation  is  carried  transversely  through 
the  whole  mass  of  the  body,  so  as  to  render  the  body  more 
or  less  clearly  divisible  into  a  series  of  parts  that  are  similarly 
composed ;  in  the  Mollusk  the  segmentation  is  limited  to  the 
shell  which  it  carries  on  its  upper  surface,  and  leaves  its 
body  as  completely  undivided  as  is  that  of  a  common  slug. 
Were  the  body  cut  through  at  each  of  the  divisions,  the  sec- 
t'on  of  it  attached  to  each  portion  of  the  shell  would  be  unlike 
all  the  other  sections.  Here  the  segmentation  has  a  purely 
functional  derivation — is  adaptive  instead  of  genetic.  The 
similarly-formed  and  similarly-placed  parts,  are  not  homolo- 
gous in  the  same  sense  as  are  the  appendages  of  a  phocnoga- 
mic  axis  or  the  limbs  of  an  insect. 

§  210.  In  studying  the  remaining  and  highest  sub-king- 
dom of  animals,  it  is  important  to  recognize  this  radical  dif- 
ference in  meaning  between  that  likeness  of  parts  which  is 
produced  by  likeness  of  modifying  forces,  and  that  likeness 
of  parts  which  is  due  to  primordial  identity  of  origin.  On 
our  recognition  of  this  difference  depends  the  view  we  take 


THE    MORPHOLOGICAL    COMPOSITION    OF    ANIMALS.         105 

of  certain  doctrines  that  have  long  been  dominant,  and  have 
still  a  wide  currency. 

Among  the  Vcrtcbrata,  as  among  the  Molhisca,  homogenesis 
is  universal.  The  two  sub-kingdoms  are  like  one  another 
and  unlike  the  remaining  sub-kingdoms  in  this,  that  in 
all  the  types  they  severally  include,  a  single  fertilized  ovum 
produces  only  a  single  individual.  It  is  true  that  as  the 
GUSTS  of  certain  Gasteropods  occasionally  exhibit  spontaneous 
hssion  of  the  vitelline  mass,  which  may  or  may  not  result  in 
the  formation  of  two  individuals ;  so  among  vertebrate  ani- 
mals we  now  and  then  meet  with  double  monsters,  which 
appear  to  imply  such  a  spontaneous  fission  imperfectly  car- 
ried out.  But  these  anomalies  serve  to  render  conspicu- 
ous the  fact,  that  in  both  these  sub-kingdoms  the  normal 
process  is  the  integration  of  the  whole  germ-mass  into  a, 
single  organism,  which  at  no  phase  of  its  development  dis- 
plays any  tendency  to  separate  into  two  or  more  parts. 

Equally  as  throughout  the  Mottmca  there  holds  throughout 
the  Vertelrata,  the  correlative  fact,  that  not  even  in  its  lowest 
any  more  than  in  its  highest  types,  is  the  body  divisible  into 
homologous  segments.  The  vertebrate  animal,  under  its 
simplest  as  under  its  most  complex  form,  is  like  the  mollusc- 
ous animal  in  this,  that  you  cannot  cut  it  into  transverse 
slices,  each  of  which  contains  a  digestive  organ,  a  respiratory 
organ,  a  reproductive  organ,  &c.  The  organs  of  the  least- 
developed  fish  as  well  as  those  of  the  most-developed 
mammal,  form  but  a  single  physiological  whole ;  and  they 
show  not  the  remotest  trace  of  having  ever  been  divisible 
into  two  or  more  physiological  wholes.  That  segmentation 
which  the  vertebrate  animal  usually  exhibits  throughout 
part  of  its  organization,  is  the  same  in  origin  and  meaning 
as  the  segmentation  of  a  Chiton's  shell ;  and  no  more  implies 
in  the  vertebrate  animal  a  composite  structure,  than  do  tho 
successive  pairs  of  branchiae  of  the  Doto  or  the  transverse  rows 
of  branchiae  in  the  Eolis,  imply  composite  structure  in  the 
molluscous  animal.  To  some  this  will  seem  a  very  question- 


106  MORPHOLOGICAL    DEVELOPMENT. 

able  proposition  ;  and  had  we  no  evidence  beyond  that  which 
adult  vertebrate  animals  of  developed  types  supply,  it  would  be 
a  proposition  not  easy  to  substantiate.  But  abundant  support 
for  it  is  to  be  found  in  the  structure  of  the  vertebrate  embryo, 
and  in  the  comparative  morphology  of  the  Vcrtebrata  in 
general. 

Embryologists  teach  us  that  the  primordial  relations  of 
parts  are  most  clearly  displayed  in  the  early  stages  of  evo- 
lution; and  that  they  generally  become  partially  or  com- 
pletely disguised  in  its  later  stages.  Hence,  were  the  verte- 
brate animal  on  the  same  level  as  the  annulose  animal  in 
degree  of  composition — did  it  similarly  consist  of  segments 
which  are  homologous  in  the  sense  that  they  are  the  prox- 
imate units  of  composition ;  we  ought  to  find  this  funda- 
mental fact  most  strongly  marked  at  the  outset.  As  in 
the  annelid-embryo,  the  first  conspicuous  change  is  the 
elongation  and  division  into  segments,  by  constrictions  that 
encircle  the  whole  body;  and  as  in  the  articulate  embryo, 
the  blastoderm  becomes  marked  out  transversely  into  pieces 
which  extend  themselves  round  the  yelk  before  the  internal 
organization  has  made  any  appreciable  progress  ;  so  in  the 
embryo  of  every  vertebrate  animal,  had  it  an  analogous  com- 
position, the  first  decided  change  should  be  a"  segmentation 
implicating  the  entire  mass.  But  it  is  not  so.  Sundry  im- 
portant differentiations  occur  before  any  divisions  begin  to 
show  themselves.  There  is  the  defining  of  that  elongated, 
elevated  area  with  its  longitudinal  groove,  which  becomes  the 
seat  of  subsequent  changes ;  there  is  the  formation  of  the 
notochord  lying  beneath  this  groove ;  there  is  the  growth 
upwards  of  the  boundaries  of  the  groove  into  the  dorsal 
laminae,  which  rapidly  develop  and  fold  over  in  the  region  of 
the  head.  Rathke,  as  quoted  and  indorsed  by  Prof.  Huxley, 
describes  the  subsequent  changes  as  follows: — "The  gelatin- 
ous investing  mass,  which,  at  first,  seems  only  to  constitute 
a  band  to  the  right  and  to  the  left  of  the  notochord,  forms 
around  it,  in  the  further  course  of  development,  a  sheath, 


THE    MOUPhui.OUlCAL    COMPOSITION    OF    ANIMALS.          107 

which  ends  in  a  point  posteriorly.  Anteriorly,  it  sends  out 
two  processes  which  underlie  the  lateral  parts  of  the  skull, 
but  very  soon  coalesce  for  a  longer  or  shorter  distance.  Pos- 
teriorly, the  sheath  projects  but  little  beyond  the  notochord ; 
but,  anteriorly,  for  a  considerable  distance,  as  far  as  the  in- 
fundibulum.  It  sends  upwards  two  plates,  which  embrace 
(.he  future  central  parts  of  the  nervous  system  laterally,  pro- 
bably throughout  their  entire  length."  All  this  precedes 
segmentation.  Considered  under  its  broadest  aspects,  the 
process  is  directly  opposed  to  the  process  among  the  An- 
^ulosa.  Whereas  among  the  Annulosa  the  first  step  is  the 
resolution  of  the  germ-mass  or  of  the  blastoderm  into  seg- 
ments, which  may  or  may  not  afterwards  become  inte- 
grated ;  in  the  Vertebrata  the  first  step  is  the  marking 
out  on  the  blastoderm  of  an  integrated  structure  within 
which  segments  subsequently  appear.  When  these  do  ap- 
pear, they  are  for  some  time  limited  to  the  middle  region  of 
the  spinal  axis ;  and  no  more  then  than  ever  after,  do  they 
implicate  the  general  mass  of  the  body  in  their  transverse  di- 
visions. On  the  contrary,  before  segmentation  has  made 
much  progress  the  rudiments  of  the  vascular  system  are  laid 
down  in  a  manner  showing  not  the  remotest  trace  of  any 
primordial  correspondence  of  its  parts  with  the  divisions  of  the 
axis.  No  less  at  variance  with  the  belief  that  the 

vertebrate  animal  is  essentially  a  series  of  homologous  parts, 
is  the  heterogeneity  which  exists  among  these  parts  on  their 
first  appearance.  Though  in  the  head  of  an  adult  articulate 
animal  there  is  little  sign  of  divisibility  into  segments  like 
those  of  the  body ;  yet  such  segments,  with  their  appropriate 
ganglia  and  appendages,  are  easily  identifiable  in  the  articu- 
late embryo.  But  in  the  vertebrata  this  antithesis  is  exactly 
reversed.  At  the  time  when  segmentation  has  become  de- 
cided in  the  dorsal  region  of  the  spine,  there  is  no  trace  of 
segments  in  'the  parts  that  are  to  form  the  skull — nothing 
whatever  to  suggest  that  the  skull  is  being  formed  out  of 
Divisions  homologous  with  vertebrae.  And  minute  observa- 


108  MORPHOLOGICAL    DEVELOPMENT. 

tion  no  more  discloses  any  such  homology  than  does  general 
appearance.  "  Remak,"  says  Prof.  Huxley,  "  has  more  fully 
proved  than  any  other  observer,  the  segmentation  into  'ur- 
wirbel,'  or  proto- vertebrae,  which  is  characteristic  of  the  ver- 
tebral column,  stops  at  the  occipital  margin  of  the  skull — 
the  base  of  which,  before  ossification,  presents  no  trace  of 
that  segmentation  which  occurs  throughout  the  vertebral 
column." 

Consider  next  the  evidence  supplied  by  comparative  mor- 
phology. In  preceding  sections  (§§  206,  208,)  it  has  been 
shown  that  among  annulose  animals,  the  divisibility  into 
homologous  parts  is  most  clearly  demonstrable  in  the  lowest 
types.  Though  in  decapodous  Crustaceans,  in  Insects,  in 
Arachnids,  there  is  difficulty  in  identifying  some  or  many  of 
the  component  somites ;  and  though  when  identified  they 
display  only  partial  correspondences  ;  yet  on  descending  to 
Annelids,  the  composition  of  the  entire  body  out  of  such 
somites  becomes  conspicuous,  and  the  homology  between  each 
somite  and  its  neighbours  is  shown  by  the  repetition  of  one 
another's  structural  details,  as  well  as  by  their  common 
gemmiparous  origin:  indeed,  in  some  cases  we  have  the 
homology  directly  demonstrated  by  seeing  a  somite  of  the 
body  transformed  into  a  head.  If,  then,  a  vertebrate  animal 
had  a  segmental  composition  of  kindred  nature,  we  ought  to 
find  it  most  clearly  marked  in  the  lowest  Vertebrata,  and 
most  disguised  in  the  highest  Vertebrata.  But  here,  as  be- 
fore, the  fact  is  just  the  reverse.  Among  the  Vertebrata  oi 
developed  type,  such  segmentation  as  really  exists  remains 
conspicuous — is  but  little  obscured  even  in  parts  of  the  spinal 
column  formed  out  of  integrated  vertebrae.  Whereas  in  the 
undeveloped  vertebrate  type,  segmentation  is  scarcely  at  all 
traceable.  The  Amphioxus,  Fig.  191,  is  not  only  without 
ossified  vertebrae ;  not  only  is  it  without  cartilaginous  re- 
presentatives of  them ;  but  it  is  even  without  anything  like 
distinct  membranous  divisions.  The  spinal  column  exists 
as  a  continuous  notochord  :  the  only  signs  of  incipient  seg- 


THE    MORPHOLOGICAL    COMPOSITION    OF    ANIMALS.         109 

mentation  being  given  by  its  membranous  sheatb,  in  the 
upper  part  of  which  "  quadrate  masses  of  somewhat  denser 


tissue  seem  faintly  to  represent  neural  spines."  Moreover, 
throughout  sundrj  groups  of  fishes  and  amphibians,  the 
segmentation  remains  very  imperfect :  only  certain  peri- 
pheral appendages  of  the  vertebra  becoming  defined  and 
solidified,  while  in  place  of  the  bodies  of  the  vertebrae  there 
still  continues  the  undivided  notochord.  Thus,  instead  of 
being  morphologically  composed  of  vertebral  segments,  the 
vertebrate  animal  in  its  primitive  form  is  entirely  without 
vertebral  segments ;  and  vertebral  segments  begin  to  appear 
only  as  we  advance  towards  developed  forms.  Once 

more,  evidence  equally  adverse  to  the  current  hypothesis 
meets  us  on  observing  that  the  differences  between  the  parts 
supposed  to  be  homologous,  are  as  great  at  first  as  at  last. 
Did  the  vertebrate  animal  primordially  consist  of  homo- 
logous segments  from  snout  to  tail ;  then  the  segments  said 
to  compose  the  skull  ought,  in  the  lowest  Vertebrata,  to  show 
themselves  much  more  like  the  remaining  segments  than 
they  do  in  the  highest  Vertebrata.  But  they  do  not.  Fishes 
have  crania  made  up  of  bones  that  are  no  more  clearly 
arrangeable  into  segments  like  vertebrae,  than  are  the  cranial 
bones  of  the  highest  mammal.  Nay,  indeed,  the  case  is 
much  stronger :  the  simplest  fish  possessing  a  skeleton, 
has  a  cranium  composed  of  cartilage  that  is  not  segmented 
at  all ! 

Besides  being  inconsistent  with  the  leading  truths  of 
Embryology  and  Comparative  Morphology,  the  hypothe- 
sis of  Goethe  and  Oken  is  inconsistent  with  itself.  The 
facts  brought  forward  to  show  that  there  exists  an  arche- 


110  MORPHOLOGICAL    DEVELOPMENT. 

typal  vertebra  ;  and  that  the  vertebrate  aniiaal  is  composed 
of  archetypal  vertebrae  arranged  in  a  series,  and  sever- 
ally modified  to  fit  their  positions — these  facts,  I  say,  so  far 
from  proving  as  much,  suffice,  when  impartially  considered, 
to  disprove  it.  No  assigned  nor  any  conceivable  attribute  of 
the  supposed  archetypal  vertebra  is  uniformly  maintained. 
The  parts  composing  it  are  constant  neither  in  their  num- 
ber, nor  in  their  relative  positions,  nor  in  their  modes  of 
ossification,  nor  in  the  separateness  of  their  several  individu- 
alities when  present  There  is  no  fixity  of  any  one  element, 
or  connexion,  or  mode  of  development,  which  justifies  even  a 
suspicion  that  vertebrae  are  modelled  after  an  ideal  pattern. 
To  substantiate  these  assertions  here  would  require  too  much 
space,  and  an  amount  of  technical  detail  wearisome  to  the 
general  reader.  The  warrant  for  them  will  be  found  in  a 
criticism  on  the  osteological  works  of  Prof.  Owen,  originally 
published  in  the  British  and  Foreign  Medico- Chirurgical 
Review  for  Oct.  1858.  This  criticism  I  add  in  the  Appendix, 
for  the  convenience  of  those  who  may  wish  to  study  the 
question  more  fully.  (See  Appendix  B.) 

Everything,  then,  goes  to  show  that  the  segmental  compo- 
sition which  characterises  the  apparatus  of  external  relation 
in  most  vertebrata,  is  not  primordial  or  genetic,  but  function- 
ally determined  or  adaptive.  Our  inference  must  be  that  the 
vertebrate  animal  is  an  aggregate  of  the  second  order,  in 
which  a  relatively  superficial  segmentation  has  been  pro- 
duced by  mechanical  intercourse  with  the  environment.  We 
shall  hereafter  see  that  this  conception  leads  us  to  a  consist- 
ent interpretation  of  the  facts — shows  us  why  there  has 
arisen  such  unity  in  variety  as  exists  in  every  vertebral 
column,  and  why  this  unity  in  variety  is  displayed  under 
countless  modifications  in  different  skeletons. 

§  211.  Glancing  back  at  the  facts  brought  together  in 
these  two  chapters,  it  seems  probable  that  there  has  gone  on 
among  animals  a  process  parallel  to  that  which  we  saw  reason 


THE    MOlirii:) LOGICAL    COMPOSITION    OF   ANIMALS.         Ill 

to  think  has  gone  on  among  plants.  Minute  aggregates  of 
those  physiological  units  which  compose  living  protoplasm, 
uxist  as  Protozoa :  some  of  them  incoherent,  indefinite,  and 
almost  homogenous  ;  and  others  of  them  more  coherent,  de- 
finite, and  heterogenous.  By  union  of  these  nucleated  parti- 
cles of  sarcode,  are  produced  various  indefinite  aggregates  of 
the  second  order — Sponges,  Thalassicollae,  Foraminifers,  &c. ; 
in  which  the  compound  individuality  is  scarcely  enough 
marked  to  subordinate  the  primitive  individualities."  But  in 
other  types,  as  the  Hydra,  the  lives  of  the  morphological 
units  are  in  a  considerable  degree,  though  not  wholly,  merged 
in  the  life  of  the  integrated  whole  they  form.  As  the  primary 
aggregate  when  it  passes  a  certain  size  undergoes  fission  or 
gemmation  ;  so  does  the  secondary  aggregate.  And  as  on 
the  lower  stage  so  on  the  higher,  we  see  cases  in  which  the 
gemmiparously-produced  individuals  part  as  soon  as  formed, 
and  other  cases  in  which  they  continue  united,  though  in  great 
measure  independent.  This  massing  of  secondary  aggregates 
into  tertiary  aggregates,  is  variously  carried  on  among  the 
Ilydrozoa,  the  A  ctinozoa,  and  the  Molluscoida.  In  most  of  the 
types  so  produced,  the  component  individualities  are  very 
little  subordinated  to  the-  individuality  of  the  mass  they  form 
— there  is  only  physical  unity  and  not  physiological  unity  ; 
but  in  certain  of  the  oceanic  Hydrozoa,  the  individuals  are  so 
far  differentiated  and  combined  as  very  much  to  mask  them. 
Forms  showing  us  clearly  the  transition  to  well-developed 
individuals  of  the  third  order,  are  not  to  be  found.  Never- 
theless, in  the  great  sub-kingdom  Anmdosa,  there  are  traits 
of  structure,  development,  and  mode  of  multiplication,  which 
go  far  to  show  that  its  members  are  such  individuals  of  the 
third  order ;  and  in  the  relations  to  external  conditions 
involved  by  the  mode  of  union,  we  find  an  adequate  cause  for 
that  obscuration  of  the  secondary  individualities  which  we 
must  suppose  has  taken  place.  The  two  other  great  sub- 
kingdoms  Mollusca  and  Vertebrata,  between  the  lower  mem- 
bers of  which  there  are  suggestive  points  of  community, 


112  MORPHOLOGICAL    DEVELOPMENT. 

present  us  only  with  aggregates  of  the  second  order,  that 
have  in  many  cases  become  very  large  and  very  complex. 
"We  find  in  them  no  trace  of  the  union  of  gemmiparously- 
produced  individuals.  Neither  the  molluscous  nor  the 
vertebrate  animal  shows  the  faintest  trace  of  a  segmenta- 
tion affecting  the  totality  of  its  structure ;  and  we  see 
good  grounds  for  concluding  that  such  segmentation  as  ex- 
ceptionally occurs  in  the  one  and  usually  occurs  in  the  other, 
IB  superinduced. 


CHAPTER  VI. 

MORPHOLOGICAL  DIFFERENTIATION   IN   PLANTS. 

§  212.  WHILE  in  the  course  of  their  evolution  plants  and 
animals  have  displayed  progressive  integrations,  there  have 
at  the  same  time  been  progressive  differentiations  of  the 
resulting  aggregates,  both  as  wholes  and  in  their  parts. 
These  differentiations  and  the  interpretations  of  them,  form 
the  second  class  of  morphological  problems. 

We  commence  as  before  with  plants.  We  have  to  con- 
sider, first,  the  several  kinds  of  modification  in  shape  they 
have  undergone ;  and,  second,  the  relations  between  thesa 
kinds  of  modification  and  their  factors.  Let  us  glance  at 
the  leading  questions  that  have  to  be  answered. 

§  213.  Irrespective  of  their  degrees  of  composition,  plants 
may,  and  do,  become  changed  in  their  general  forms.  Are 
their  changes  capable  of  being  formulated?  The  inquiry 
which  meets  us  at  the  outset  is — does  a  plant's  shape  admit 
of  being  expressed  in  any  universal  terms  ? — terms  that 
remain  the  same  for  all  genera,  orders,  and  classes. 

After  plants  considered  as  wholes,  have  to  be  considered . 
their  proximate  components,  which  vary  with  their  degrees 
of  composition,  and  in  the  highest  plants  are  what  we  call 
branches.  Is  there  any  law  traceable  among  the  contrasted 
shapes  of  different  branches  in  the  same  plant  ?  Do  the  rela- 
tive developments  of  parts  in  the  same  branch  conform  to 


114  MOKI'HOI.OGICAL    DEVELOPMENT. 

nny  law  ?  And  are  these  laws,  if  they  exist,  allied  with  one 
another  and  with  that  to  which  the  shape  of  the  whole 
plant  conforms  ? 

Descending  to  the  components  of  these  components,  which 
in  developed  plants  we  distinguish  as  leaves,  there  meet  us 
kinlred  questions  respecting  their  relative  sizes,  their  rela- 
tive shapes,  and  their  shapes  as  compared  with  those  of 
foliar  organs  in  general.  Of  their  morphological  differentia- 
tions, also,  it  has  to  be  asked  whether  they  exemplify  any 
truth  that  is  exemplified  by  the  entire  plant  and  by  its  larger 
parts. 

Then,  a  step  lower,  we  come  down  to  those  morphological 
units  of  which  leaves  and  fronds  consist ;  and  concerning 
these  arise  parallel  inquiries  touching  their  divergencies 
from  one  another  and  from  cells  in  general. 

Tne  problems  thus  put  together  in  several  groups  can- 
not of  course  be  rigorously  separated.  Evolution  pre-sup- 
poses  transitions  which  make  all  such  classings  more  or  less 
conventional;  and  adherence  to  them  must  be  subordinata 
to  the  needs  of  the  occasion. 

§  214.  In  studying  the  causes  of  the  morphological 
differentiations  thus  grouped  and  prospectively  generalized, 
we  shall  have  to  bear  in  mind  several  orders  of  forces  which 
it  will  be  well  briefly  to  specify. 

Growth  tends  inevitably  to  initiate  changes  in  the 
s^iape  of  any  aggregate,  by  changing  both  the  amounts  of 
the  incident  forces  and  the  forces  which  the  parts  exert  on 
one  another.  With  the  mechanical  actions  this  is  obvious  : 
mitter  that  is  sensibly  plastic  cannot  be  increased  in  mass 
without  undergoing  a  change  in  its  proportions,  consequent 
on  the  diminished  ratio  of  its  cohesive  force  to  the  force  of 
gravitation.  With  the  physiological  actions  it  is  equally 
obvious :  increase  of  size,  other  things  equal,  alters  the 
relations  of  the  parts  to  the  material  and  dynamical  factors 
of  nutrition ;  and  by  so  affecting  differently  the  nutrition 


MORPHOLOGICAL   DIFFERENTIATION    IX    PLANTS.  115 

of   different    parts,    initiates   further    changes    of    propor- 
tions. 

Similarly  in  any  composite  plant,  the  proximate  units  as 
fast  as  they  accumulate  are  subjected  to  mutual  influences 
that  are  unlike  one  another  and  are  continually  changing. 
The  earlier- formed  units  become  mechanical  supporters  of  the 
later- formed  units,  and  so  experience  modifying  forces  from 
which  the  later-formed  units  are  exempt.  Further,  these 
elder  units  simultaneously  begin  to  serve  as  channels  through 
which  materials  are  carried  to  and  from  the  younger  units — 
another  cause  of  differentiation  that  goes  on  increasing  in  in- 
tensity. Once  more,  there  arise  ever-strengthening  contrasts 
between  the  amounts  of  light  which  full  upon  the  youngest  or 
outermost  units  and  the  eldest  or  innermost  units  ;  whence 
result  structural  contra-sts  of  yet  another  kind.  Evidently, 
then,  along  with  the  progressive  integration  of  cells  into 
fronds,  of  fronds  into  axes,  and  of  axes  into  plants  still  more 
composite,  there  come  into  play  sundry  causes  of  differen- 
tiation which  act  on  the  whole  and  on  each  of  its  parts, 
whatever  their  grade.  The  forces  to  be  overcome,  the  forces 
to  be  utilized,  and  the  matters  to  be  appropriated,  do  not 
remain  the  same  in  their  proportions  and  modes  of  action  for 
any  two  members  of  the  aggregate :  be  they  members  of  the 
first,  second,  third,  or  any  other  order. 

§  215.  Nor  are  these  the  only  kinds  and  causes  of  hetero- 
geneity which  we  have  to  consider.  Beyond  the  more 
general  changes  produced  in  the  relative  sizes  and  shapes  of 
plants  and  their  parts  by  progressive  aggregation,  there  are 
the  more  particular  changes  determined  by  the  more  particu- 
lar conditions. 

Plants  as  wholes  assume  unlike  attitudes  towards  their  en- 
vironments ;  they  have  many  ways  of  articulating  their 
parts  with  one  another ;  they  have  many  ways  of  adjusting 
their  parts  towards  surrounding  agencies.  These  are  causes  of 
special  differentiations  additional  to  those  general  differentia- 


116  MORPHOLOGICAL   DEVELOPMENT. 

tions  that  result  from  increase  of  mass  and  increase  of  com- 
position. In  each  part  considered  individually,  there  arises 
a  characteristic  shape  consequent  on  that  relative  position 
towards  external  and  internal  forces,  which  the  mode  of 
growth  entails.  Every  member  of  the  aggregate  presents 
itself  in  a  more  or  less  peculiar  way  towards  the  light,  towards 
the  air,  and  towards  its  point  of  support ;  and  according  to 
the  relative  homogeneity  or  heterogeneity  in  the  incidence  of 
the  agencies  thus  brought  to  bear  on  it,  will  be  the  relative 
homogeneity  or  heterogeneity  of  its  shape. 

§  216.  Before  passing  from  this  a  priori  view  of  the  mor- 
phological differentiations  which  necessarily  accompany 
morphological  integrations,  to  an  a  posteriori  view  of  them,  it 
seems  naedful  to  specify  the  meanings  of  certain  descriptive 
terms  we  shall  have  to  employ. 

Taking  for  our  broadest  division  among  forms,  the  regular 
and  the  irregular,  we  may  divide  the  latter  into  those  which 
are  wholly  irregular  and  those  which,  being  but  partially 
irregular,  suggest  some  regular  form  to  which  they  approach. 
By  slightly  straining  the  difference  between  them,  two  current 
words  may  be  conveniently  used  to  describe  these  subdivi- 
sions. The  entirely  irregular  forms  we  may  class  as 
asymmetrical — literally  as  forms  without  any  equality  of 
dimensions.  The  forms  which  approximate  towards  regu- 
larity without  reaching  it,  we  may  distinguish  as  unsym- 
metrical — a  word  which,  though  it  asserts  inequality  of 
dimensions,  has  been  associated  by  use  rather  with  such 
slight  inequality  as  constitutes  an  observable  departure  from 
equality. 

Of  the  regular  forms  there  are  several  classes,  differing  in 
the  number  of  directions  in  which  equality  of  dimensions  is 
repeated.  Hence  results  the  need  for  names  by  which  sym- 
metry of  several  kinds  may  be  expressed. 

The  most  regular  of  figures  is  the  sphere  :  its  dimensions 
are  the  same  from  centre  to  surface  in  all  directions ;  and  if 


MORPHOLOGICAL    DIFFERENTIATION    IN    PLANTS.  117 

cut  by  any  plane  through  the  centre,  the  separated  parts  are 
equal  and  similar.  This  is  a  kind  of  symmetry  which  stands 
alone,  and  will  be  hereafter  spoken  of  as  spherical  symmetry. 

When  a  sphere  passes  into  a  spheroid,  either  prolate  or  ob- 
late, there  remains  but  one  set  of  planes  that  will  divide  it 
into  halves  which  are  in  all  respects  alike  ;  namely,  the 
planes  in  which  its  axis  lies,  or  which  have  its  axis  for  their 
line  of  intersection.  Prolate  and  oblate  spheroids  may 
severally  pass  into  various  forms  without  losing  this  pro- 
perty. The  prolate  spheroid  may  become  egg-shaped  or  py- 
riform,  and  it  will  still  continue  capable  of  being  divided  into 
two  equal  and  similar  parts  by  any  plane  cutting  it  down 
its  axis  ;  nor  will  forming  constrictions  round  it  deprive  it 
of  this  property.  Similarly  with  the  oblate  spheroid.  The 
transition  from  a  slight  oblateness  like  that  of  an  orange 
to  an  oblateness  reducing  it  nearly  to  a  flat  disc,  does  not 
alter  its  divisibility  into  like  halves  by  every  plane  passing 
through  its  axis.  And  clearly  the  moulding  of  any  such 
flattened  oblate  spheroid  into  the  shape  of  a  plate,  leaves  it 
as  before,  symmetrically  divisible  by  all  planes  at  right 
angles  to  its  surface  and  passing  through  its  centre.  This 
species  of  symmetry  is  called  radial  symmetry.  It  is  familiarly 
exemplified  in  such  flowers  as  the  daisy,  the  tulip,  and  the 
dahlia. 

From  spherical  symmetry,  in  which  we  have  an  infinite 
number  of  axes  through  each  of  which  may  pass  an  infinite 
number  of  planes  severally  dividing  the  aggregate  into  equal 
and  similar  parts ;  and  from  radial  symmetry,  in  which  we 
have  a  single  axis  through  which  may  pass  an  infinite  num- 
ber of  planes  severally  dividing  the  aggregate  into  equal  and 
similar  parts  ;  we  now  turn  to  bilateral  symmetry,  in  which 
the  divisibility  into  equal  and  similar  parts  becomes  very 
limited.  Noting,  for  the  sake  of  completeness,  that  there  is 
a  sextuple  bilateralness  in  the  cube  and  its  derivative  forms, 
which  admit  of  division  into  equal  and  similar  parts  by  planes 
passing  through  the  three  diagonal  axes  and  by  planes  passing 


118  MORPHOLOGICAL    DEVELOPMENT. 

through  the  three  axes  that  join  the  centres  of  the  surfaces, 
let  us  limit  our  attention  to  the  three  kinds  of  bilateralness 
which  here  concern  us.  The  first  of  these  is  triple 

bilateral  symmetry.  This  is  the  symmetry  of  a  figure  having 
three  axes  at  right  angles  to  one  another,  through  each  of 
which  there  passes  a  single  plane  that  divides  the  aggregate 
into  corresponding  halves.  A  common  brick  will  serve  as  an 
example ;  and  of  objects  not  quite  so  simple,  the  most  familiar 
is  that  modern  kind  of  spectacle-case  which  is  open  at  both 
ends.  This  may  be  divided  into  corresponding  halves  along 
its  longitudinal  axis,  by  cutting  it  through  in  the  direction 
of  its  thickness  or  by  cutting  it  through  in  the  direction  of 
its  breadth  ;  or  it  may  be  divided  into  corresponding  halves 
by  cutting  it  across  the  middle.  Of  objects  which 

illustrate  double  bilateral  symmetry,  may  be  named  one  of 
those  boats  built  for  moving  with  equal  facility  in  either  di- 
rection, and  therefore  made  alike  at  stem  and  stern.  Ob- 
viously such  a  boat  is  separable  into  equal  and  similar  parts 
by  a  vertical  plane  passing  through  stem  and  stern  ;  and  it  is 
also  separable  into  equal  and  similar  parts  by  a  vertical  plane 
cutting  it  amidships.  To  exemplify  single  bilateral 

symmetry  it  needs  but  to  turn  to  the  ordinary  boat  of  which 
the  two  ends  are  unlike.  Here  there  remains  but  the  one 
plane  passing  vertically  through  stem  and  stern,  on  the  op- 
posite sides  of  which  the  parts  are  symmetrically  disposed. 

These  several  kinds  of  symmetry  as  placed  in  the  fore- 
going order,  imply  increasing  heterogeneity.  The  greatest 
uniformity  in  shape  is  shown  by  the  divisibility  into  like 
parts  in  an  infinite  number  of  infinite  series  of  ways  ;  and 
the  greatest  degree  of  multiformity  consistent  with  any 
regularity,  is  shown  by  the  divisibility  into  like  parts  in 
o:ily  a  single  way.  Hence,  in  tracing  up  organic  evolution 
as  displayed  in  morphological  differentiations,  we  may  ex- 
pect to  pass  from  the  one  extreme  of  spherical  symmetry, 
to  the  other  extreme  of  single  bilateral  symmetry.  This 
expectation  we  shall  find  to  be  completely  fulfilled. 


CHAPTER 


THE  GENERAL  SHAPES  OF  PLANTS. 

§  217.  AMONG  protophytes  those  which  are  by  general 
consent  regarded  as  the  simplest,  are  the  Protococci.  As 
shown  in  Fig.  1,  they  are  globular  cells  presenting  no  ob- 
vious differentiation  save  that  between  inner  and  outer  parts. 
Their  uniformity  of  figure  coexists  with  a  mode  of  life  involv- 
ing the  uniform  exposure  of  all  their  sides  to  incident  forces. 
The  Protococcus  nimlis,  which  colours  red  the  snow  through 
which  it  spreads  with  such  marvellous  rapidity,  is  subject  to 
no  constant  contrasts  in  the  amounts  of  light,  heat,  air,  or 
moisture,  on  its  upper  and  lower  surfaces.  For  though  each 
individual  may  have  its  external  parts  differently  related  to 
environing  agencies,  yet  the  new  individuals  produced  by 
spontaneous  fission  have  no  means  of  maintaining  parallel 
relations  of  position  among  their  parts.  On  the  contrary, 
the  indefiniteness  of  the  attitudes  into  which  successive 
generations  fall,  must  prevent  the  rise  of  any  unlikeness  be- 
tween one  portion  of  the  surface  and  another.  Spherical 
symmetry  continues  because,  on  the  average  of  cases,  inci- 
dent forces  are  equal  in  all  directions. 

Other   orders   of    ProtopJiyta  have   much   more   special 

forms,  along  with  much  more  special  attitudes  :   their  ho- 

mologous parts  maintaining,  from  generation  to  generation, 

unlike  relations  to  incident  forces.     The  Desmidiacece  and 

VOL.  II.  6 


120  MORPHOLOGICAL   DEVELOPMENT. 

Dtatomacece,  of  which.  Figs.  2  and  3  show  examples,  severally 
include  genera  characterized 
by  triple  bilateral  sym- 
metry. A  Namcula  is  di- 
visible into  corresponding 
halves  by  a  transverse  plane 
and  by  two  longitudinal  planes — one  cutting  its  valves  at 
right  angles  and  the  other  passing  between  its  valves.  The 
like  is  true  of  those  numerous  transversely-constricted  forms 
of  DesmidiacecB,  exemplified  by  the  second  of  the  individuals 
represented  in  Fig.  2.  If  now  we  ask  how  a  Navicula  is  re- 
lated to  its  environment,  we  see  that  its  mode  of  life  exposes 
it  to  three  different  sets  of  forces :  each  set  being  resolvable 
into  two  equal  and  opposite  sets.  A  Namcula  moves  in  the 
direction  of  its  length,  with  either  end  foremost.  Hence,  on 
the  average,  its  ends  are  subject  to  like  actions  from  the 
agencies  to  which  its  motions  subject  it.  Further,  either 
end  while  moving,  exposes  its  right  and  left  sides  to  amounts 
of  influence  which  in  the  long  run  must  be  equal.  If,  then, 
the  two  ends  are  not  only  like  one  another,  but  have  cor- 
responding right  and  left  sides,  the  symmetrical  distribu- 
tion of  parts  answers  to  the  symmetrical  distribution  of 
forces.  Passing  to  the  two  edges  and  the  two  flat  sur- 
faces, we  similarly  find  a  clue  to  their  likenesses  and  differ- 
ences in  their  respective  relations  to  the  things  around  them. 
These  locomotive  protophytes  move  through  the  entangled 
masses  of  fragments  and  fibres  produced  by  decaying  organ- 
isms and  confervoid  growths.  The  interstices  in  such  matted 
accumulations  are  nearly  all  of  them  much  longer  in  one 
dimension  than  in  the  rest — form  crevices  rather  than 
regular  meshes.  Hence,  a  small  organism  will  have  much 
greater  facility  of  insinuating  itself  through  this  debris,  in 
which  it  finds  nutriment,  if  its  transverse  section  is  flattened 
instead  of  square  or  circular.  And  while  we  see  how,  by 
survival  of  the  fittest,  a  flattened  form  is  likely  to  be  ac- 
quired by  diatoms  having  this  habit ;  we  also  see  that  like- 


THE  GENERAL  SHAPES  OF  PLANTS.          121 

ness  will  be  maintained  between  the  two  flat  surfaces  and 
between  the  two  edges.  For,  on  the  average,  the  relations 
of  the  two  flat  surfaces  to  the  sides  of  the  openings  through 
which  the  diatom  passes,  will  be  alike ;  and  so,  too,  on  the 
average,  will  be  the  relations  of  the  two  edges.  In 

desmids  of  the  type  exemplified  by  the  second  individual  in 
Fig.  2,  a  kindred  equalization  of  dimensions  is  otherwise  in- 
sured. There  is  nothing  to  keep  one  of  the  two  surfaces 
uppermost  rather  than  the  other ;  and  hence,  in  the  long 
succession  of  individuals,  the  two  surfaces  are  sure  to.  ba 
similarly  exposed  to  light  and  agencies  in  general.  When 
to  this  is  added  the  fact  that  spontaneous  fission  occurs 
transversely  in  a  constant  way,  it  becomes  manifest  that  the 
two  ends,  while  they  are  maintained  in  conditions  like  one 
another,  are  maintained  in  conditions  unlike  those  of  the  two 
edges.  Here  then,  as  before,  triple  bilateral  symmetry  in 
form,  coexists  with  a  triple  bilateral  symmetry  in  the 
average  distribution  of  actions. 

Still  confining  our  attention  to  aggregates  of  the  first 
order,  let  us  next  note  what  results  when  the  two  ends  are 
permanently  subject  to  different  conditions.  The  fixed 
unicellular  plants,  of  which  examples  are  given  in  Figs.  4,  5, 
and  6,  beverally  illustrate  the  contrast  in  shape  that  arises 


between  the  part  that  is  applied  to  the  supporting  surface 
and  the  part  that  extends  into  the  surrounding  medium. 
These  two  parts  which  are  the  most  unlike  in  their  relations 
to  incident  forces,  are  the  most  unlike  in  their  forms.  Ob- 


122  MORPHOLOGICAL   DEVELOPMENT. 

serve,  next,  that  the  part  which  lifts  itself  into  the  water  01 
air,  is  more  or  less  decidedly  radial.  Each  upward  growing 
tubule  of  C odium  adhcerens,  Fig.  4,  has  its  parts  disposed 
with  some  regularity  around  its  axis ;  the  upper  stem  and 
spore-vessel  of  Hydrogastrum,  Fig.  5,  display  a  lateral 
growth  that  is  approximately  equal  in  every  direction ;  and 
the  branches  of  the  Botrytis,  Fig.  6,  shoot  out  with  an  ap- 
proach to  evenness  on  all  sides.  Plants  of  this  low  type 
are  naturally  very  variable  in  their  modes  of  growth  :  each 
individual  being  greatly  modified  in  form  by  its  special  cir- 
cumstances. But  they  nevertheless  show  us  a  general  like- 
ness between  parts  exposed  to  like  forces,  as  well  as  a  general 
unlikeness  between  parts  exposed  to  unlike  forces. 

Respecting  the  forms  of  these  aggregates  of  the  first  order, 
it  has  only  to  be  added  that  they  are  asymmetrical  where 
there  is  total  irregularity  in  the  incidence  of  forces.  We 
have  an  example  in  the  indefinitely  contorted  and  branched 
shape  of  a  fungus-cell,  growing  as  a  mycelium  among  the 
particles  of  soil  or  through  the  interstices  of  organic  tissue. 

§  218.  Re- illustrations  of  the  general  truths  which  the 
forms  of  these  vegetal  aggregates  of  the  first  order  display, 
are  furnished  by  vegetal  aggregates  of  the  second  order. 
The  equalities  and  inequalities  of  growth  in  different  direc- 
tions, prove  to  be  similarly  related  to  the  equalities  and  in- 
equalities of  environing  actions  in  different  directions. 

Of  spherical  symmetry,  an  instance  occurs  in  the  Volvost 
globalor.  The  ciliated  cells,  here  so  united  as  to  produce  a 
small,  mulberry-shaped,  hollow  ball,  cause,  by  the  movements 
of  their  cilia,  a  simultaneous  rotation  of  the  ball  and  pro- 
gress of  it  through  the  water.  There  is  nothing  to  de- 
termine the  axis  of  rotation  or  the  direction  of  rotation. 
And  if  the  axis  and  direction  of  rotation  continually  vary, 
as  we  may  conclude  that  they  do,  then  the  different  mem- 
bers of  the  aggregate  severally  occupy  in  their  turns  like 
positions  towards  surrounding  agencies ;  and  so  are  not 


THE    GENERAL   SHAPES   OF    PLANTS.  123 

made  to  lose  their  homogeneity  of  form  and  distribution. 
Vegetal  aggregates  of  the  second  order  are  usually  fixed : 
locomotion  is  exceptional.  Fixity  implies  that  the  surface 
of  attachment  is  differently  circumstanced  from  the  free  sur- 
face. Hence  we  may  expect  to  find,  as  we  do  find,  that 
among  these  rooted  aggregates  of  the  second  order,  as  among 
those  of  the  first  order,  the  primary  contrast  of  shape  is 
between  the  adherent  part  and  the  loose  part.  Sea  -weeds 
variously  exemplify  this.  In  some  the  fronds  are  very 
irregular  and  in  some  tolerably  regular ;  in  some  the  form  is 
pseudo-foliar  and  in  some  pseud-axial ;  but  differing  though 
they  do  in  these  respects,  they  agree  in  having  the  end 
which  is  attached  to  a  solid  body  unlike  the  other  end.  The 
same  truth  is  seen  in  such  secondary  aggregates  as  the  com- 
mon fungi,  or  rather  in  their  immensely-developed  organs  of 
fructification.  A  puff-ball,  Fig.  192,  presents  no  other 
obvious  unlikeness  of  parts  than  that  between  its  under  and 
upper  surfaces.  So  too  with  the  stalked  kinds  that  frequent 
our  woods  and  pastures.  In  the  types  which  Figs.  193, 
194,  195,  delineate,  the  unlikenesses  between  the  rooted 
ends  and  the  expanded  ends,  as  well  as  between  the  under 
and  upper  surfaces  of  the  expanded  ends,  are  obviously 
related  to  this  fundamental  contrast  of  conditions.  Nor  is 
this  relation  less  clearly  displayed  in  the  sessile  fungi  which 
grow  out  from  the  sides  of  trees,  as  shown  at  a,  b,  Fig. 
196.  That  which  is  common  to  this  and  the  preceding  types, 
;.s  the  contrast  between  the  attached  end  and  the  free 
end. 

From  what  these  forms  have  in  common,  let  us  turn 
to  that  which  they  have  not  in  common,  and  observe  the 
causes  of  the  want  of  community.  A  puff-ball  shows  us 
in  the  simplest  way,  the  likeness  of  parts  accompanying 
likeness  of  conditions,  along  with  the  unlikeness  of  parts 
accompanying  unlikeness  of  conditions.  For  while,  if  we 
cut  vertically  through  its  centre,  we  find  a  difference  be- 
*ween  top  and  bottom,  if  we  cut  horizontally  through  its 


124 


MORPHOLOGICAL    DEVELOPMENT. 


centre,  we  find  no  differences  among  its  several  sides. 
Being,  on  the  average  of  cases,  similarly  related  to  the  envi- 
ronment all  round,  it  remains  the  same  all  round.  The 
radial  symmetry  of  the  mushroom  and  other  vertically- 


/5»2 
/^~ > 


growing  fungi,  illustrates  this  connexion  of  cause  and 
effect  still  better.  But  now  mark  what  happens  in  the 
group  of  Agaricus  zylophilus,  shown  in  Fig.  195.  Radi- 
ally symmetrical  as  is  the  type,  and  radially  symmetri- 
cal as  are  those  centrally-placed  individuals  which  are 
equally  crowded  all  round,  we  see  that  the  peripheral  indi- 
viduals, dissimilarly  circumstanced  on  their  outer  sides  and 
on  their  sides  next  the  group,  have  partially  changed  their 
radial  symmetry  into  bilateral  symmetry.  It  is  no  longer 
possible  to  make  two  corresponding  halves  by  any  vertical 
plane  cutting  down  through  the  pileus  and  the  stem ;  but 
there  is  only  one  vertical  plane  that  will  thus  produce  cor- 
responding halves — the  plane  on  the  opposite  sides  of  which 
the  relations  to  the  environment  are  alike.  And  then  mark 
that  the  divergence  from  all- sided  symmetry  towards  two- 
sided  symmetry,  here  caused  in  the  individual  by  special 
circumstances,  is  characteristic  of  the  race  where  the 
habits  of  the  race  constantly  involve  two-sidedness  of  condi- 
tions. Besides  being  exemplified  by  such  comparatively 
undifferentiated  types  as  Boletus,  Fig.  196,  a,  b,  this  truth 
is  exemplified  by  members  of  the  genus  just  named.  In 
Agaricus  horizon-tails  y  Fig.  196,  <?,  we  have  a  departure  from 
radial  symmetry  that  is  conspicuous  only  in  the  form  of  the 
stem.  A  more  decided  bilateralness  exists  in  A.  palmatus, 


THE   GENERAL    SHAPES    OF    PLANTS.  12<3 

shown  in  elevation  at  d  and  in  section  at  (F.  And  A.  flabelli- 
fonim,  of  which,  e  and  e  are  different  views,  exhibits  com- 
plete bilateralness — a  bilateralness  in  which  there  is  the 
greatest  likeness  of  the  parts  that  are  most  similarly  condi- 
tioned, and  the  greatest  unlikeness  of  the  parts  that  are  most 
dissimilarly  conditioned. 

Among  plants  of  the  second  order  of  composition,  it  will 
suffice  to  note  one  further  class  of  facts  which  are  the  con- 
verse of  the  foregoing  and  have  the  same  implications.  These 
are  the  facts  showing  that  along  with  habitual  irregularity  in 
the  relations  to  external  forces,  there  is  habitual  irregularity 
in  the  mode  of  growth.  Besides  finding  such  facts  among 
Thallogens,  as  in  the  tubers  of  underground  fungi  and  in  the 
creeping  films  of  sessile  lichens,  which  severally  show  us 
variations  of  proportions  obviously  caused  by  variations  in  the 
amounts  of  the  influences  on  their  different  sides,  we  also 
among  Acrogens  of  inferior  types,  find  irregularities  of  form 
along  with  irregularities  in  environing  actions.  The  fronds 
of  the  Harchantiacew  or  such  Jungermanniacece  as  are  shown  in 
Figs.  41,  42,  43,  illustrate  the  way  in  which  each  lowly-or- 
gaiiized  aggregate  of  the  second  order,  not  individuated  by 
the  mutual  dependence  of  its  parts,  has  its  form  determined 
by  the  balance  of  facilities  and  resistances  which  each  portion 
of  the  frond  meets  with  as  it  spreads. 

§  219.  Among  plants  that  display  integration  of  the  third 
degree,  and  among  plants  still  further  compounded,  these 
same  truths  are  equally  manifest.  In  the  forms  of  such 
plants  we  see  primary  contrasts  and  secondary  contrasts, 
which,  no  less  clearly  than  the  foregoing,  are  related  to 
contrasts  of  conditions. 

That  flowering  plants  from  the  daisy  up  to  the  oak,  have 
in  common  the  fundamental  unlikeness  between  the  upward 
growing  part  and  the  downward  growing  part ;  and  that 
this  most  marked  unlikeness  corresponds  with  the  most  mark- 
ed unlikeness  between  the  two  parts  of  their  environment 


123  MORPHOLOGICAL   DEVELOPMENT. 

soil  and  air ;  are  facts  too  conspicuous  to  be  named  were  they 
not  important  items  in  the  argument.     More  instructive, 
perhaps,  because  less  familiar,  is  the  fact  that  we  miss  this 
extreme  contrast  in  flowering  plants  which  have  not  their 
higher  and  lower  portions  exposed  to  conditions  so  extremely 
contrasted.     A  parasite  like  the  Dodder,  growing  in  entan- 
gled masses  upon  other  plants,  from  which  it  sucks  the  juices, 
is  not  thus  divisible  into  two  strongly-distinguished  halves. 
Leaving  out  of  consideration  the  difference  between  the 
supporting  part  and  the  supported  part  in  phaenogams,  and 
looking  at  the  supported  part  only,  we  observe  between  its 
form  and  the  habitual  incidence  of  forces,  a  relation  like  that 
which  we  observed  in  the  simpler  plants.     Phaenogams  that 
are  practically  if  not  literally  uniaxial,  and  those  which  de- 
velop their  lateral  axes  only  in  the  shape  of  axillary  flowers, 
when  uninterfered  with  ordinarily  send  up  vertical  axes  round 
which  the  leaves  and  flowers  are  disposed  with  a  more  or  less 
decided   radial   symmetry.     Gardens   and  fields   supply  us 
with  such  instances  as  the  Tulip  and  the  Orchis ;  and  on  a 
larger  scale  the  Palms  and  the  Aloes  are  fertile  in  examples. 
The  exceptions,  too,  are  instructive.     Besides  the  individual 
divergences  that  arise  from  special  interferences,  there  are 
to  be  traced  general  divergences  where  the  habits  of  the 
plants  expose   them   to   general   interferences  in   anything 
approaching  to  constant  ways.     Plants  which,  like  the  Fox- 
glove, have  spikes  of  flowers  that  are  borne  on  flexible  foot- 
stalks, have  their  flowers  habitually  bent  round  to  one  face  of 
the  stem :  an  unlikeness  of  distribution  probably  caused  by 
unlikeness  in  the  relation  to  the  sun's  rays.     The  wild  Hya- 
cinth, too,  with  stem  so  flexible  that  its  upper  part  droops, 
shows  us  how  a  consequent  difference  in  the  action  of  gravity 
on  the  flowers,  causes  them  to  deviate  from  their  typically 
radial  arrangement  towards  a  bilateral  arrangement. 

Much  more  conspicuous  are  the  segeneral  and  special  re- 
lations of  form  to  general  and  special  actions  in  the  environ- 
ment, among  phscnogams  that  are  multiaxial.  That  when 


THE    GENERAL    SHAPES    OF    PLANTS.  127 

standing  alone,  and  in  positions  where  the  winds  do  not  injure 
them  or  adjacent  objects  shade  them,  shrubs  and  trees  develop 
with  tolerable  evenness  on  all  sides,  is  an  obvious  truth.  Equal- 
ly obvious  is  the  truth  that,  when  growing  together  in  a  wood, 
and  mutually  interfered  with  on  all  sides,  trees  still  show 
obscurely  radial  distributions  of  parts ;  though,  under  such 
conditions,  they  have  tall  taper  stems  with  branches  directed 
upwards — a  difference  of  shape  clearly  due  to  the  different 
incidence  of  forces.  And  almost  equally  obvious  is  the  truth, 
that  a  tree  of  this  same  kind  growing  at  the  edge  of  the  wood, 
has  its  outer  branches  well-developed  and  its  inner  branches 
comparatively  ill-developed.  Fig.  197,  which  very  inaccur- 


ately  represents  this  difference,  will  serve  to  make  it  manifest 
that  while  one  of  the  peripheral  trees  can  be  cut  into  some- 
thing like  two  similar  halves  by  a  vertical  plane  directed  to- 
wards the  centre  of  the  wood — a  plane  on  each  side  of  which 
the  conditions  are  alike — it  cannot  be  cut  into  similar  halves 
by  any  other  plane.  A  like  divergence  from  an  indefinitely- 
radial  symmetry  towards  an  indefinitely-bilateral  symmetry, 
occurs  in  trees  that  have  their  conditions  made  bilateral  by 
growing  on  inclined  surfaces.  Two  of  the  common  forms 
observable  in  such  cases  are  given  in  Fig.  198.  Here  there 
is  divisibility  into  parts  that  are  tolerably  similar,  by  a  vertical 
plane  running  directly  down  the  hill ;  but  not  by  any  other 
plane.  Then,  further,  there  is  the  bilateralness,  similar  in 
general  meaning  though  differently  caused,  which  we  see 
in  trees  exposed  to  strong  prevailing  winds.  Almost  every 


128  MORPHOLOGICAL   DEVELOPMENT. 

sea-coast  has  abundant  examples  of  stunted  trees  which,  like 
the  one  shown  in  Fig.  199,  have  been  made  to  deviate  from 
their  ordinary  equal  growth  on  all  sides  of  a  vertical  axis,  to 
a  growth  that  is  equal  only  on  the  opposite  sides  of  a  vertical 
plane  directed  towards  the  wind's  eye. 

From  among  vegetal  aggregates  of  the  third  order,  we  have 
now  only  to  add  examples  of  the  entirely  asymmetrical  form 
that  accompanies  an  entirely  irregular  distribution  of  inci- 
dent forces.  Creeping  plants  furnish  such  examples.  They 
show  us,  alike  when  climbing  up  vertical  or  inclined  surfaces 
or  trailing  along  the  ground,  that  their  branches  grow  hither 
and  thither  as  the  balance  of  forces  aids  or  opposes  ;  and  the 
general  outline  is  without  symmetry  of  any  kind,  because 
the  environing  influences  have  no  kind  of  regularity  in  their 
arrangement. 

§  220.  Along  with  some  unfamiliar  facts,  I  have  here  set 
down  facts  that  are  so  familiar  as  to  seem  scarcely  worth 
noting.  It  is  because  these  facts  have  become  meaningless 
to  perceptions  deadened  by  infinite  repetitions  of  them,  that 
it  is  needful  here  to  point  out  their  meaning.  Not  alone  for 
its  intrinsic  importance,  has  the  unlikeness  between  the 
attached  ends  and  the  free  ends  been  traced  among  plants 
of  all  degrees  of  integration.  Nor  is  it  simply  because  of  the 
significance  they  have  in  themselves,  that  instances  have  been 
given  of  those  different  varieties  of  symmetry  and  asymmetry 
which  the  free  ends  of  plants  equally  display :  be  they  plants 
of  the  first,  second,  third,  or  any  higher  order.  Neither  has 
the  only  other  purpose  been  that  of  showing  how,  in  the  radial 
symmetry  of  some  vegetal  aggregates  and  the  single  bilateral 
symmetry  of  others,  there  are  traceable  the  same  ultimate 
principles  as  in  the  spherical  symmetry  and  triple  bilateral 
symmetry  of  certain  minute  plants  first  described.  But  the 
main  object  has  been  to  present  under  their  simplest  aspects, 
those  general  laws  of  morphological  differentiation  which  are 
fulfilled  by  the  component  parts  of  each  plant. 


THE   GENERAL   SHAPES   OF   PLANTS.  129 

If  organic  form  is  determined  by  the  distribution  of  forces, 
and  the  approach  in  every  case  towards  an  equilibrium  of 
inner  actions  with  outer  actions  ;  then  this  relation  between 
forms  and  forces  must  hold  alike  in  the  organism  as  a  whole, 
in  its  proximate  units,  and  in  its  units  of  lower  orders.  Formu- 
la s  which  express  the  shapes  of  entire  plants  in  terms  of  sur- 
rounding conditions,  must  be  formulas  which  also  express 
the  shapes  of  their  several  parts  in  terms  of  surrounding 
conditions.  If,  therefore,  we  find  that  a  plant  as  a  whole  is 
radially  symmetrical  or  bilaterally  symmetrical  or  asymme- 
trical, according  as  the  incident  forces  affect  it  equally  on  all 
sides  of  an  axis  or  affect  it  equally  only  on  the  opposite  sides 
of  one  plane  or  affect  it  equally  in  no  two  directions ;  then, 
we  may  expect  that  each  member  of  a  plant  will  display  radial 
symmetry  where  environing  influences  are  alike  along  many 
radii,  bilateral  symmetry  where  there  is  bilateralness  of 
environing  influences,  and  unsymmetry  or  asymmetry  where 
there  is  partial  or  entire  departure  from  a  balance  of  sur- 
rounding actions. 

To  show  that  this  expectation  is  borne  out  by  the  facts, 
will  be  the  object  of  the  following  four  chapters.  Let  us 
begin  with  the  largest  parts  into  which  plants  are  divisible ; 
and  proceed  to  the  successively  smaller  parts. 


CHAPTER  VIII. 

THE   SHAPES   OF   BRANCHES. 

§  221.  AGGREGATES  of  the  first  order  supply  a  few  examples 
of  forms  ramified  in  an  approximately-regular  manner,  under 
conditions  which  subject  their  parts  to  approximately-regu- 
lar distributions  of  forces.  Some  unicellular  Algce,  becoming 
elaborately  branched,  assume  very  much  the  aspects  of  small 
trees  ;  and  show  us  in  their  branches  analogous  relations  of 
forms  to  forces.  Bryopsis  plumosa  may 
be  instanced.  Fig.  200  represents  the 
end  of  one  of  its  lateral  ramifications, 
above  and  beneath  which  come  others  of 
like  characters.  Here  it  will  be  seen  that 
the  attached  and  free  ends  differ ;  that 
the  two  sides  are  much  alike  ;  and  that  they  are  unlike  the 
upper  and  under  surfaces,  which  resemble  one  another. 

§  222.  Fig.  201  shows  us  how  in  an  aggregate  of  the  se- 
cond order,  each  proximate  component  is 
modified  by  its  relations  to  the  rest ;  just 
as  we  before  saw  a  whole  fungus  of  the 
same  type  modified  by  its  relations  to  en- 
vironing objects.  If  a  branch  of  the  fun- 
gus here  figured,  be  compared  with  one  of 
the  fungi  clustered  together  in  Fig.  195, 
or,  still  better,  with  one  of  the  laterally- 


THE  SHAPES  OF  BRANCHES. 


131 


growing  fungi  shown  in  Fig.  196,  there  will  be  perceived  a 
kindred  transition  from  radial  to  bilateral  symmetry,  occurring 
under  kindred  conditions.  The  portion  of  the  pileus  next  to 
the  side  of  attachment  is  undeveloped  in  this  branched  form 
as  in  the  simple  form ;  and  in  the  one  case  as  in  the  other, 
the  stem  is  modified  towards  the  side  of  attachment.  A  di- 
vision into  similar  halves,  which,  as  shown  in  Fig.  196  e,  might 
be  made  of  the  whole  fungus  by  a  vertical  plane  passing 
through  the  centre  of  the  pileus  and  the  axis  of  the  support- 
ing body,  might  here  be  made  of  the  branch,  by  a  vertical 
plane  passing  through  the  centre  of  its  pileus  and  the  axis  of 
the  main  stem.  Among  aggregates  of  this  order,  the  Alga?. 
furnish  cases  of  kindred  nature.  In  the  branches  of  Lessonia, 
Fig.  37,  may  be  observed  a  substantially- similar  relationship  : 
their  inner  parts  being  less  developed  than  their  outer  parts, 
while  their  two  sides  are  developed  in  approximately  equal 
degrees,  they  are  rendered  bilateral. 

§  223.  These  few  cases  introduce  us  to  the  more  familiar 
but  more  complex  cases  which  plants  of  the  third  degree  of 
aggregation  present.  At  a,  b,  c,  Fig.  202,  are  sketched  three 


homologous  parts  of  the  same  tree :  a  being  the  leading 
shoot;  I  a  lateral  branch  near  the  top,  and  c  a  lateral 
branch  lower  down.  There  is  here  a  double  exemplifica- 
tion. "While  the  branch  a,  as  a  whole,  has  its  branchleta 


132  MORPHOLOGICAL   DEVELOPMENT. 

arranged  with  tolerable  regularity  all  round,  in  corre« 
spondence  with  its  equal  exposure  on  all  sides,  each  branch- 
let  shows  by  its  curve  as  much  bilateral  symmetry  as 
its  simple  form  permits.  The  branch  b,  dissimilarly 
circumstanced  on  the  side  next  the  main  stem  and  on 
the  side  away  from  it,  has  an  approximate  bilateralness 
as  a  whole,  while  the  bilateralness  of  its  branchlets  varies 
with  their  respective  positions.  And  in  the  branch  c,  having 
its  parts  still  more  differently  conditioned,  these  traits  of 
structure  are  still  more  marked.  Extremely  strong  contrasts 
of  this  kind  occur  in  trees  having  very  regular  modes  ot 
growth.  The  uppermost  branches  of  a  Spruce-fir  have  radially 
arranged  branchlets :  each  of  them,  if  growing  vigorously, 
repeats  the  type  of  the  leading  shoot,  as  shown  in  Fig.  203, 
a,  b.  But  if  we  examine  branches  lower  and  lower  down  the 
tree,  we  find  the  vertically-growing  branchlets  bear  a  less  and 
less  ratio  to  the  horizontally- growing  ones ;  until,  towards  the 
bottom,  the  radial  arrangement  has  wholly  merged  into  the 
bilateral.  Shaded  and  confined  by  the  branches  above  them, 
these  eldest  branches  develop  their  offshoots  in  those  direc- 
tions where  there  is  most  space  and  light :  becoming  finally 
quite  flattened  and  fan-shaped,  as  shown  at  Fig  203,  c.  And 
on  remembering  that  each  of  these  eldest  branches,  when  first 
it  diverged  from  the  main  stem,  was  radial,  we  see  not  only 
that  between  the  upper  and  lower  branches  does  this  contrast 
in  structure  hold,  but  also  that  each  branch  is  transformed 
from  the  radial  to  the  bilateral  by  the  progressive  change  in 
its  environment.  Other  forces  besides  those  which  aid 

or  hinder  growth,  conspire  to  produce  this  two-sided  character 
in  lateral  branches.  The  annexed  Fig.  204,  sketched  from 
an  example  of  the  Pinus  Coulterii  at  Kew,  shows  very  clearly 
how,  by  mere  gravitation,  the  once  radially-arranged  branch- 
lets  may  be  so  bent  as  to  produce  in  the  branch  as  a  whole  a 
decided  bilateralness.  A  full-grown  Araucaria,  too,  exhibits 
in  its  lower  branches  modifications  similarly  caused ;  and  in 
each  of  such  branches  there  may1>e  remarked  the  further  fact, 


TILE    SHAPES    OF    BRANCHES.  133 

jhat  its  upward-bending  termination  has  a  partially-modified 
radialness,  at  the  same  time  that  its  drooping  lateral  branch- 
lets  give  to  the  part  nearer  the  trunk  a  completely  bilateral 
character. 

Now  in  these  few  instances,  which  are  typical  of  countless 
instances  that  might  be  given,  we  see,  as  we  saw  in  the  case 
of  the  fungi,  that  the  same  thing  is  true  of  the  parts  in 
their  relations  to  the  whole  and  to  one  another,  which  is  true 
of  the  whole  in  its  relations  to  the  environment  at  large. 
Entire  trees  become  bilateral  instead  of  radial,  when  exposed 
to  forces  that  are  equal  only  on  opposite  sides  of  one  plane ; 
and  in  their  branches,  parallel  changes  of  form  occur  under 
parallel  changes  of  conditions. 

§  224.  There  remains  to  be  said  something  respecting  the 
distribution  of  leaves.  How  a  branch  carries  its  leaves  con- 
stitutes one  of  its  characters  as  a  branch ;  and  is  to  be  con- 
sidered apart  from  the  characters  of  the  leaves  themselves. 
The  principles  hitherto  illustrated  we  shall  here  find  illus- 
trated still  further. 

The  leading  shoot  and  all  the  upper  twigs  of  a  fir-tree, 
have  their  pin- shaped  leaves  evenly  distributed  all  round,  or 
placed  radially  ;*  but  as  we  descend,  we  find  them  beginning 
to  assume  a  bilateral  distribution ;  and  on  the  lower,  hori- 
zontally-growing branches,  their  distribution  is  quite  bilateral. 
Between  the  Irish  and  English  kinds  of  Yew,  there  is  a  con- 
trast of  like  significance.  The  branches  of  the  one,  shooting 
up  as  they  do  almost  vertically,  are  clothed  with  leaves 
all  round  ;  while  those  of  the  other,  which  spread  laterally, 
bear  their  leaves  on  the  two  sides.  In  trees  with  better- 
developed  leaves,  the  same  principle  is  more  or  less  manifest 
in  proportion  as  the  leaves  are  more  or  less  enabled  by  their 
structures  to  maintain  fixed  positions.  Where  the  foot-stalks 

*  Here  and  throughout,  the  word  radial  is  applied  equally  to  the  spiral  and 
the  whorled  structures.  These,  as  being  alike  on  all  sides,  are  similarly  distin- 
guished from  arrangements  that  are  alike  on  two  sides  only. 


134  MORPHOLOGICAL    DEVELOPMENT. 

are  long  and  slender,  and  where,  consequently,  each  leaf,  ac- 
cording to  its  weight,  the  flexibility  and  twist  of  its  foot- 
stalk, and  the  direction  of  the  branch  it  grows  from,  falls 
into  some  indefinite  attitude,  the  relations  are  obscured.  But 
where  the  foot-stalks  are  stiff,  as  in  the  Laurel,  it  will  be 
found,  as  before,  that  from  the  topmost  and  upward-growing 
branches  the  leaves  diverge  on  all  sides ;  while  the  under- 
most branches,  growing  out  from  the  shade  of  those  above, 
have  their  leaves  so  turned  as  to  bring  them  into  rows  hori- 
zontally spread  out  on  the  two  sides  of  each  branch. 

A  kindred  truth,  having  like  implications,  comes  into  view 
when  we  observe  the  relative  sizes  of  leaves  on  the  same 
305-  branch,    where    their    sizes    differ. 

Fig.  205  represents  a  branch  of  a 
Horse-chesnut,  taken  from  the  low- 
ermost fringe  of  the  tree,  where  the 
light  has  been  to  a  great  extent  in- 
tercepted from  all  but  the  most  pro- 
truded parts.  Beyond  the  fact  that 
the  leaves  are  bilaterally  distributed 
on  this  drooping  branch,  instead  of 
being  distributed  symmetrically  all  round,  as  on  one  of  the 
ascending  shoots,  we  have  here  to  note  the  fact  that  there 
is  unequal  development  on  the  upper  and  lower  sides.  Each 
of  the  compound  leaves  acquires  a  foot-stalk  and  leaflets  that 
are  large  in  proportion  to  the  supply  of  light ;  and  hence,  as 
we  descend  towards  the  bottom  of  the  tree,  the  clusters  of 
leaves  display  increasing  contrasts.  How  marked  these  con- 
trasts become  will  be  seen  on  comparing  a  and  b,  which  form 
one  pair  of  leaves  that  are  normally  equal,  or  c  and  d,  which 
form  another  pair  normally  equal. 

Let  us  not  omit  to  note,  while  we  have  this  case  before  us, 
the  proof  it  affords  that  these  differences  of  development  are 
in  a  considerable  degree  determined  by  the  different  con- 
ditions of  the  parts  after  they  have  been  unfolded.  Though 
those  inequalities  of  dimensions  whence  the  differentiations 


THE  SHAPES  OF  BRANCHES.  135 

of  form  result,  are  in  many  cases  largely  due  to  tlie  inequali- 
ties in  the  circumstances  of  the  parts  while  in  the  bud  (which 
are  however  representative  of  inequalities  in  ancestral  cir- 
cumstances) ;  yet  these  are  clearly  not  the  sole  causes  of  the 
unlikenesses  that  eventually  arise.  For  the  leaf-buds  whence 
the  larger  leaves  in  Fig.  205  were  developed,  instead  of  being 
at  first  more  favourably  circumstanced  than  the  others,  were 
less  favourably  circumstanced.  So  that  this  bilateralness 
that  results  from  the  unequal  sizes  of  the  leaves,  must  be  con- 
sidered as  wholly  due  to  the  differential  actions  that  come  into 
play  after  the  leaves  have  assumed  their  typical  structures. 

§  225.  How  in  the  arrangement  of  their  twigs  and  leaves, 
branches  tend  to  lapse  from  forms  that  are  approximately 
symmetrical  to  forms  that  are  quite  asymmetrical,  need  not 
be  demonstrated :  it  is  sufficiently  conspicuous.  But  it  may 
be  well  to  point  out  how  the  tendency  to  do  this  further 
enforces  our  argument.  The  comparatively  regular  budding- 
out  of  secondary  axes  and  tertiary  axes,  does  not  usually 
produce  an  aggregate  which  maintains  its  regularity,  for 
the  simple  reason  that  many  of  the  axes  abort.  Terminal 
buds  are  some  of  them  destroyed  by  birds ;  others  are  bur- 
rowed into  by  insects ;  others  are  nipped  by  frost ;  others 
are  broken  off  or  injured  during  gales  of  wind.  The  envi- 
ronment of  each  branch  and  its  branchlets  is  thus  ever 
being  varied  on  all  sides :  here,  space  being  left  vacant  by 
the  death  of  some  shoot  that  would  ordinarily  have  occupied 
it ;  and  there,  space  being  trenched  on  by  the  lateral  growth 
of  some  adjacent  branch  that  has  had  its  main  axis  broken. 
Hence  the  asymmetry  or  heterogeneity  of  form  which  the 
branch  assumes,  is.  caused  by  the  asymmetrical  distribution 
of  incident  forces — a  result  and  a  cause  that  go  on  ever  com- 
plicating. 

§  226.  One  conspicuous  trait  in  the  shapes  of  branches 
has  still  to  be  named.  Their  proximal  or  attached  ends 
differ  from  their  distal  or  free  ends,  in  the  same  way  that 


136  MORFIIOI.IGICAL   DEVELOPMENT. 

the  lower  ends  of  trees  differ  from  their  upper  ends.  This 
fact,  like  the  fact  to  which  it  is  here  paralleled,  has  had  its 
significance  obscured  by  its  extreme  familiarity.  But  it 
shows  in  a  striking  way  how  the  most  differently-conditioned 
parts  become  the  most  strongly  contrasted  in  their  struc- 
tures. A  phaenogamic  axis  is  made  up  of  homologous 
segments,  marked  off  from  one  another  by  the  nodes ;  and 
a  compound  branch  consists  of  groups  of  such  segments.  The 
earliest- formed  segments,  alike  of  the  tree  and  of  each 
branch,  serve  as  mechanical  supports  and  channels  for  sap 
to  the  successive  generations  of  segments  that  grow  out  of 
them ;  and  become  more  and  more  shaded  by  their  pro- 
geny as  these  increase.  Hence  the  progressively-increasing 
contrasts.  If  the  trunk  of  a  tree  were  sawn  horizontally 
into  a  series  of  slabs,  each  some  two  inches  thick  or  there- 
abouts ;  if  each  of  the  main  branches  were  similarly  divided 
transversely,  and  the  like  were  done  with  all  the  branches 
borne  by  it,  down  to  their  ultimate  twigs,  which  would  be  se- 
verally cut  across  at  each  internode ;  then,  morphologically 
considered,  any  one  of  these  slabs  would  be  the  homologue 
of  any  internode  of  an  ultimate  twig,  with  its  leaf  and  axil- 
lary bud.  In  the  immense  contrast  between  these  oldest 
and  youngest  units  of  composition,  we  should  have  exhibited 
the  cumulative  result  of  continuous  differentiation  caused  by 
continuous  action  of  modifying  forces — the  one  unit  having 
been  originally  just  like  the  other. 

§  227.  Thus,  then,  it  is  with  the  proximate  parts  of  plants  as 
it  is  with  plants  as  wholes.  The  radial  symmetry,  the  bilateral 
symmetry,  and  the  asymmetry,  which  branches  display  in 
different  trees,  in  different  parts  of  the  same  tree,  and  at 
different  stages  of  their  own  growths,  prove  to  be  all  conse- 
quent on  the  ways  in  which  they  stand  towards  the  entire 
plexus  of  surrounding  actions.  The  principle  that  the 
growths  are  unequal  in  proportion  as  the  relations  to  the 
environment  are  unequal,  serves  to  explain  all  the  leading 
traits  of  structure. 


CHAPTER  IX. 

THE    SHAPES   OF  LEAVES. 

§  228.  NEXT  in  the  descending  order  of  composition  come 
compound  leaves.  The  relative  sizes  and  distributions  of 
their  leaflets,  as  affecting  their  forms  as  wholes,  have  to  be 
considered  in  their  relations  to  conditions.  Figs.  206,  207, 
represent  leaves  of  the  common  Oxalis  and  of  the  Marsilea, 
in  which  radial  symmetry  is  as  completely  displayed  as  the 
small  number  of  leaflets  permits.  This  equal  development 
of  the  leaflets  on  all  sides,  occurs  where  the  foot-stalks,  grow- 
ing up  vertically  from  creeping  or  underground  stems,  are 
so  long  that  the  leaves  either  do  not  interfere  with  one 
another  or  do  it  in  an  inconstant  way  :  the  leaflets  are  not 
differently  conditioned  on  different  sides,  as  they  are  where 
the  foot-stalks  grow  out  in  the  ordinary  manner.  How  un- 
likeness  of  position  influences  the  leaflets  is  clearly  shown  in 
a  Clover-leaf,  Fig.  208,  which  deviates  from  the  Oxalis-leaf 
but  slightly  towards  bilateralness,  as  it  deviates  from  it  but 
slightly  in  the  attitude  of  its  petiole  ;  which  is  a  little  in- 
clined away  from  the  others  borne  by  the  same  procumbent 
axis.  A  familiar  example  of  an  almost-radial  symmetry 
along  with  almost  equal  relations  to  surrounding  conditions, 
occurs  in  the  root-leaves  of  the  Lupin,  Fig.  209,  b.  Here 
though  we  have  lateral  divergence  from  a  vertical  axis,  yet 
the  long  foot-stalks  preserve  nearly  erect  positions,  and 
carry  their  leaves  to  such  distances  from  the  axis,  that  the 


133 


MORPHOLOGICAL    DEVELOPMENT. 


development  of  the  leaflets  on  the  side  next  the  axis  is  not 
ranch  hindered.  Still  the  interference  of  the  leaves  with  one 
another  is,  on  the  average,  somewhat  greater  on  the  proximal 
side  than  on  the  distal  side ;  and  hence  the  interior  leaflets 
are  rather  less  than  the  exterior  leaflets.  In  further  proof  of 
which  influence,  let  it  be  added  that,  as  shown  in  the  figure, 
at  a,  the  leaves  growing  out  of  the  flowering-stem  devi- 
ate towards  the  two-sided  form  more  decidedly.  Two- 
sidedness  is  much  greater  where  there  is  a  greater  relative 
proximity  of  the  inner  leaflets  to  the  axis,  or  where  the  foot- 
stalk approaches  towards  a  horizontal  position.  The  Horse- 
chesnut,  Fig.  205,  already  instanced  as  showing  how  the 
arrangements  and  sizes  of  leaves  are  determined  by  the 
incidence  of  forces,  serves  also  to  show  how  the  incidence 
of  forces  determines  the  relative  sizes  and  arrangements 
of  leaflets.  Fig.  210,  which  shows  a  leaf  of  the 


267 


Bombax,  further  illustrates  this  relation  of  structure  to  con- 
ditions. 

Compound  leaves  that  are  completely  bilateral,  present  us 
with  modifications  of  form  exemplifying  the  same  general 
truth  in  another  way.  In  them  the  proximal  and  distal 
parts  have  none  of  that  resemblance  which  we  see  in  those 
intermediate  forms  just  described :  the  portion  next  the  axis 
and  the  portion  furthest  from  the  axis  are  entirely  different ; 
and  the  only  likeness  is  lietween  the  wings  or  leaflets  on 


THE  SHAPES  OF  LEAVES.  139 

opposite  sides  of  the  main  foot-stalk  or  midrib.  On  turning 
back  to  Fig.  65,  it  will  be  seen  that  the  compound  leaf  there 
drawn  to  exemplify  another  truth,  serves  also  to  exemplify  this 
truth :  the  homologous  parts  a,  b,  c,  d,  while  they  are  unlike 
one  another,  are,  in  their  main  proportions,  severally  like 
the  parts  with  which  they  are  paired.  And  here  let  us  not 
overlook  a  characteristic  which  is  less  conspicuous  but  not 
less  significant.  Each  of  the  lateral  wings  has  winglets 
that  are  larger  en  the  one  side  than  on  the  other ;  and  in 
each  case  the  two  sides  are  dissimilarly  conditioned.  Even 
in  the  several  components  of  each  wing  may  be  traced  a  like 
divergence  from  symmetry,  along  with  a  like  inequality  in 
the  relations  to  the  rest :  the  proximal  half  of  each  leaflet 
is  habitually  larger  than  the  distal  half.  In  the  leaves  of 
the  Bramble,  previously  figured,  kindred  facts  are  presented. 
How  far  such  differences  of  development  are  due  to  the  posi- 
tions of  the  parts  in  the  bud ;  how  far  the  respective 
spaces  available  for  the  parts  when  unfolded  affect  them ; 
and  how  far  the  parts  are  rendered  unlike  by  unlikenesses  in 
their  relations  to  light ;  it  is  difficult  to  say.  Probably 
these  several  factors  operate  in  all  varieties  of  proportion. 
That  the  habitual  shading  of  some  parts  by  others  largely 
aids  in  causing  these  divergences  from  symmetry,  is  very 
instructively  shown  by  the  compound  leaves  of  the  Cow- 
parsnip.  Fig.  211  represents  one  of  these.  While  the  leaf  as  a 


whole  is  bilaterally  symmetrical,  each  of  the  wings  has  an  un- 
Bymmetrical  bilateralness :  the  side  next  the  axis  being  larger 
than  the  remoter  side.  How  does  this  happen  ?  Fig.  212, 


140  MORPHOLOGICAL   DEVELOPMENT 

which  is  a  diagrammatic  section  down  the  midrib  of  the 
leaf,  showing  its  inclined  attitude  and  the  positions  of  the 
wings  a,  I,  c,  will  make  the  cause  clear.  As  the  wings 
overlap  like  the  bars  of  a  Venetian  blind,  each  intercepts 
some  light  from  the  one  below  it ;  and  the  one  below  it 
thus  suffers  more  on  its  distal  side  than  on  its  proximal  side. 
Hence  the  smaller  development  of  the  distal  side.  That  this 
is  the  cause  is  further  shown  by  the  proportion  that  is  main- 
tained between  the  degree  of  obscuration  and  the  degree  of 
non-development ;  for  this  unlikeness  is  greater  between  the 
two  sides  a  and  a',  than  between  b  and  b',  or  c  and  c',  at  the 
same  time  that  the  interference  is  greater  in  the  lower  wings 
than  in  the  upper.  Of  course  in  this  case  and  in  the  kindred 
cases  hereafter  similarly  interpreted,  it  is  not  meant  that  this 
differentiation  is  consequent  solely,  or  even  chiefly,  on 
the  differential  actions  experienced  by  the  individual  plant. 
Though  there  is  good  reason  to  believe  that  the  rate  of  growth 
in  each  part  of  each  leaf  is  affected  by  the  incidence  of  light, 
yet  contrasts  so  marked  and  so  systematic  as  these  are  not 
explicable  without  taking  into  account  the  inheritance  of 
modifications  either  functionally  caused  or  caused  by  spon- 
taneous variation.  Clearly,  the  tendency  will  be  towards 
the  preservation  of  a  plant  which  distributes  its  chlorophyll 
in  the  most  economical  way ;  and  hence  there  will  always  be 
a  gravitation  towards  a  form  in  which  shaded  parts  of  leaves 
are  undeveloped. 

§  229.  From  compound  leaves  to  simple  ones,  we  find 
transitions  in  leaves  of  which  the  divisions  are  partial  in- 
stead of  total ;  and  in  these  we  see,  with  equal  clearness,  the 
relations  between  forms  and  positions  that  have  been  traced 
thus  far.  Fig.  213  is  the  leaf  of  a  "Winter-aconite,  in  which, 
round  a  vertical  petiole,  there  is  a  radial  distribution  of  half- 
sepai-ated  leaflets.  The  Cecropia-leaf,  Fig.  214,  shows  us  a 
two-sided  development  of  the  parts  beginning  to  modify, 
but  not  obliterating,  the  all-sided  arrangement ;  and  this 


sun 

Los 


THE    SHAPES    OF    LEAVES. 


141 


mixed  symmetry  occurs  under  conditions  that  are  interme- 
diate. A  more  marked  degree  of  the  same  relation  is  pre- 
sented in  the  leaf  of  the  Lady's  Mantle,  Fig.  215.  And 


then  in  the  Sycamore  and  the  Vine,  we  have  a  cleft  type  of 
leaf  in  which  a  decided  bilateralness  of  form  co-exists  with 
a  decided  bilateralness  of  conditions. 

The  quite  simple  leaves  to  which  we  now  descend,  exhibit, 
very  distinctly,  a  parallel  series  of  facts.  "Where  they  grow 
up  on  long  and  completely-independent  foot-stalks,  without 
definite  subordination  to  some  central  vertical  axis,  the 
leaves  of  water-plants  are  symmetrically  peltate.  Of  this 
the  sacred  Indian-bean,  Fig.  216,  furnishes  an  example.  Here 
there  is  only  a  trace  of  bilateralness  in  the  venation  of  the 
leaf,  corresponding  to  the  very  small  difference  of  the  con- 
ditions on  the  proximal  and  distal  sides.  In  the  Victoria 
regia,  Fig.  217,  the  foot-stalks,  though  radiating  almost 
horizontally  from  a  centre,  are  so  long  as  to  keep  the  leaves 
quite  remote  from  one  another ;  and  in  it  each  leaf  is  almost 
symmetrically  peltate,  with  a  bilateralness  indicated  only  by 
a  seam  over  the  line  of  the  foot-stalk.  The  leaves  of  the 
Nymphxa,  Fig.  218,  more  closely  clustered,  and  having  less 
317  z*s 


room  transversely  than  longitudinally,  exhibit  a  marked 
advance  to  the  two-sided  form ;  not  only  in  the  excess  of 
the  length  over  the  breadth,  but  in  the  existence  of  a  cleft, 


142 


MORPHOLOGICAL    DEVELOPMENT, 


where  in  the  Victoria  regia  there  is  merely  a  seam.  Among 
land-plants  similar  forms  are  found  under  analogous  condi- 
tions. The  common  Hydrocotyle,  Fig.  219,  which  sends 


up  direct  from  its  roots  a  few  almost  upright  leaf-stalks,  has 
these  surmounted  by  peltate  leaves ;  which  leaves,  however, 
diverge  slightly  from  radial  symmetry  in  correspondence  with 
the  slight  contrast  of  circumstances  which  their  grouping  in- 
volves. Another  case  is  supplied  by  the  Nasturtium,  Fig. 
220,  which  combines  the  characters — a  creeping  stem,  long 
leaf-stalks  growing  up  at  right  angles  to  it,  and  unsymme- 
trically  peltate  leaves,  of  which  the  least  dimension  is,  on 
the  average,  towards  the  stem.  But  perhaps  the  most 
striking  illustration  is  that  furnished  by  the  Cotyledon  umbi- 
licus, Fig.  221,  in  which  different  kinds  of  syinmstry  occur 
in  the  leaves  of  the  same  plant,  along  with  differences  in  their 
relations  to  conditions.  The  root-leaves,  a,  that  grow  up  on 
vertical  petioles  before  the  flower-stalk  makes  its  appearance, 
are  symmetrically  peltate ;  while  the  leaves  which  subse- 
quently grow  out  of  the  flower-stalk,  b,  are  at  the  bottom 
transitionally  bilateral,  and  higher  up  completely  bilateral. 

That  the  bilateral  form  of  leaf  is  the  ordinary  form, 
corresponds  with  the  fact  that,  ordinarily,  the  circum- 
stances of  the  leaf  are  different  in  the  direction  of  the  plant's 
axis  from  what  they  are  in  the  opposite  direction,  while 


THE    SHAPES   OF    LEAVES.  14.'3 

transversely  the  circumstances  are  alike.  It  is  needless  to 
give  diagrams  to  illustrate  this  extremely  familiar  truth. 
Whether  they  are  broad  or  long,  oval  or  heart-shaped,  pointed 
or  obtuse,  the  leaves  of  most  trees  and  plants  will  be  remem- 
bered by  all  as  having  the  ends  by  which  they  are  attached 
unlike  the  free  ends,  while  the  two  sides  are  alike.  And  it  will 
also  be  remembered  that  these  equalities  and  inequalities  of 
development  correspond  with  the  equalities  and  inequalities 
in  the  incidence  of  forces. 

§  230.  A  confirmation  that  is  interesting  and  important, 
is  furnished  by  the  cases  in  which  leaves  present  unsymme- 
trical  forms  in  positions  where  their  parts  are  unsymmetri- 
cally  related  to  the  environment.  A  considerable  deviation 
from  bilateral  symmetry  may  be  seen  in  a  leaf  which  habitu- 
ally so  carries  itself,  that  the  half  on  the  one  side  of  the  midrib 
is  more  shaded  than  the  other  half.  The  drooping  branches  of 
the  Lime,  exemplified  in  Fig.  222,  show  us  leaves  so  arranged 


J 


and  so  modified.  On  examining  their  attitudes  and  their 
relations  one  to  another,  it  will  be  found  that  each  leaf  is  so 
inclined  that  the  half  of  it  next  the  shoot  grows  over  the 
shoot  and  gets  plenty  of  light  ;  while  the  other  half  so  hangs 
down  that  it  comes  a  good  deal  into  the  shade  of  the  pre- 
ceding leaf.  The  result  is  that  having  leaves  which  fall  into 
these  positions,  the  species  profits  by  a  large  development  of 
the  exposed  halves  ;  and  by  survival  of  the  fittest  acting 
along  with  the  direct  effect  of  extra  exposure,  this  modifi- 
cation becomes  established.  How  unquestionable  is  the 
connexion  between  the  relative  positions  of  the  halves  and 
their  relative  developments,  will  be  admitted  on  observing  a 
VOL.  II,  T 


144  MORPHOLOGICAL    DEVELOPMENT. 

converse  case.  Fig.  223  represents  a  shoot  of  Goldfussia 
glomcrata.  Here  the  leaves  are  so  set  on  the  stem  that  the 
inner  half  of  each  leaf  is  shaded  by  the  subsequently-formed 
leaf,  while  its  outer  half  is  not  thus  shaded ;  and  here  we  find 
the  inner  half  less  developed  than  the  outer  half.  But  the 
most  conclusive  evidence  of  this  relation  between  unsymme- 
trical  form  and  unsymmetrical  distribution  of '  surrounding 
forces,  is  supplied  by  the  genus  Begonia  ;  for  in  it  we  have 
a  manifest  proportion  between  the  degree  of  the  alleged 
effect  and  the  degree  of  the  alleged  cause.  These  plants 
produce  their  leaves  in  pairs,  in  such  a  way  that  the  connate 
leaves  interfere  with  one  another,  much  or  little  according 
as  the  foot-stalks  are  short  or  long ;  and  the  result  is  a  cor- 
relative divergence  from  symmetry.  In  Begonia  nelumbice- 
folia,  which  has  petioles  so  long  that  the  connate  leaves  are  not 
kept  close  together,  there  is  but  little  deviation  from  a  bilate- 
rally-peltate form ;  whereas,  accompanying  the  compara- 
tively marked  and  constant  proximity  in  B.  pruinata,  Fig. 
224,  we  see  a  more  decidedly  unsymmetrical  shape  ;  and  in 
B.  mahringii,  Fig.  225,  the  modification  thus  caused  is 
pushed  so  far  as  to  destroy  the  peltate  structure.* 

§  231.  Again,  then,  we  are  taught  the  same  truth.  Here, 
as  before,  we  see  that  homologous  units  of  any  order  become 

*  We  may  note  that  some  of  these  leaves,  as  those  of  the  Lime,  furnish  indica- 
tions of  the  ratio  which  exists  between  the  effects  of  individual  circumstances  and 
those  of  typical  tendencies.  On  the  one  hand,  the  leaves  borne  by  these  drooping 
oranches  of  the  Lime  are  with  hardly  an  exception  unsymmetrical  more  or  less 
decidedly,  even  in  positions  where  the  causes  of  unsymmetry  are  not  in  action :  a 
fact  showing  us  the  repetition  of  the  type  irrespective  of  the  conditions.  On  the 
other  hand,  the  degree  of  deviation  from  symmetry  is  extremely  variable,  even  on 
the  same  shoot :  a  fact  proving  that  the  circumstances  of  the  individual  leaf  are 
highly  influential  in  modifying  its  form.  But  the  most  striking  evidence  of  this 
direct  modification  is  afforded  by  the  suckers  of  the  Lime.  Growing,  as  these 
do,  in  approximately  upright  attitudes,  the  leaves  they  bear  do  not  stand  to  one 
another  in  the  way  above  described,  and  the  causes  of  unsymmetry  are  not  in 
action  ;  and  here,  though  there  is  a  general  leaning  to  the  unsymmetrical  form, 
a  large  proportion  of  the  leaves  become  quite  symmetrical 


THE  SHAPES  OF  LEAVES.  145 

differentiated  in  proportion  as  their  relations  to  incident 
forces  become  different.  And  here,  as  before,  we  see  that  in 
each  unit,  considered  by  itself,  the  differences  of  dimension 
are  greatest  in  those  directions  in  which  the  parts  are  most 
differently  conditioned ;  while  there  are  no  differences  be- 
tween the  dimensions  of  the  parts  that  are  not  differently 
conditioned.* 

*  It  was  by  an  observation  on  the  forms  of  Itaves,  that  I  was  first  led  to  the 
views  set  forth  in  the  preceding  and  succeeding  chapters  on  the  morphological 
differentiation  of  plants  and  animals.  In  the  year  1851,  during  a  country 
ramble  in  which  the  structures  of  plants  had  been  a  topic  of  conversation  with  a 
friend— Mr  G.  H.  Lewes — I  happened  to  pick  up  the  leaf  of  a  buttercup,  and 
drawing  it  by  its  foot-stalk  through  my  fingers  so  as  to  thrust  together  its  deeply- 
cleft  divisions,  observed  that  its  palmate  and  almost  radial  form  was  changed 
into  a  bilateral  one ;  and  that  were  the  divisions  to  grow  together  in  this  new 
position,  an  ordinary  bilateral  leaf  would  result.  Joining  this  observation  with 
the  familiar  fact  that  leaves,  in  common  with  the  larger  members  of  plants, 
habitually  turn  themselves  to  the  light,  it  occurred  to  me  that  a  natural  change 
in  the  circumstances  of  the  leaf  might  readily  cause  such  a  modification  of  form  as 
that  which  I  had  produced  artificially.  If,  as  they  often  do  with  plants,  soil 
and  climate  were  greatly  to  change  the  Labit  of  the  buttercup,  making  it 
branched  and  shrub-like ;  and  if  these  palmate  leaves  were  thus  much  over- 
shadowed by  each  other;  would  not  the  inner  segments  of  the  leaves  grow 
towards  the  periphery  of  the  plant  where  the  light  was  greatest,  and  so  change 
the  palmate  form  into  a  more  decidedly  bilateral  form  ?  Immediately  I  began  to 
look  round  for  evidence  of  the  relation  between  the  forms  of  leaves  and  the  general 
characters  of  the  plants  they  belonged  to  ;  and  soon  found  some  signs  of  con- 
nexion. Certain  anomalies,  or  seeming  anomalies,  however,  prevented  me  from 
then  pursuing  the  inquiry  much  further.  But  consideration  cleared  np  these 
difficulties;  and  the  idea  afterwards  widened  into  the  general  doctrine  here 
elaborated.  Occupation  with  other  things  prevented  me  from  giving  expression 
to  this  general  doctrine  until  Jan.  1859  ;  when  I  published  an  outline  of  it  in 
the  Medico -Chirurgical  Review <. 


CHAPTER  X. 

THE   SHAPES    OF   FLOWERS. 

|  232.  FOLLOWING  an  order  like  that  of  preceding  chap- 
ters, let  us  first  note  a  few  typical  facts  respecting  the  forms 
of  clusters  of  flowers,  apart  from  the  forms  of  the  flowers  them- 
selves. Two  kindred  kinds  of  Leguminosce  will  serve  to  show 
how  the  members  of  clusters  are  distributed  in  an  all-sided  man- 
ner or  in  a  two-sided  manner,  according  as  the  circumstances 
are  alike  on  all  sides  or  alike  on  only  two  sides.  In  Hippo- 
crepis,  represented  in  Fig.  226,  the  flowers  growing  at  the  end 
of  a  vertical  stem,  are  arranged 
round  it  in  radial  symmetry. 
Contrariwise  in  Melilotus,  Fig. 
227,  where  the  axillary  stem 
bearing  the  flowers  is  so 
placed  in  relation  to  the  main 
stem,  that  its  outer  and  inner 
sides  are  differently  condi- 
tioned, the  flowers  are  all  on 
the  outer  side  :  the  cluster  is 
bilaterally  symmetrical,  since 
it  may  be  cut  into  approx- 
imately equal  and  similar 
groups  by  a  vertical  plane  passing  through  the  main  axis. 

Plants  of  this  same  tribe  furnish  clusters  of  intermediate 
characters  having  intermediate  conditions.  Among  these, 
as  among  the  clusters  whicn  other  types  present,  may  bo 


THE    SHAPES   OF    FLOWERS.  147 

found  some  in  which  conformity  to  the  general  law  is  not 
obvious.  The  discussion  of  these  apparent  anomalies  would 
carry  us  too  much  out  of  our  course.  A  clue  to  the  explana- 
tion of  them  will,  I  believe,  be  found  in  the  explanation 
presently  to  be  given  of  certain  kindred  anomalies  in  the 
forms  of  individual  flowers. 

§  233.  The  radially-symmetrical  form  is  common  to  all 
individual  flowers  that  have  vertical  axes.  In  plants  which 
are  practically  if  not  literally  uniaxial,  and  bear  their  flowers 
at  the  ends  of  upright  stalks,  so  that  the  faces  open  hori- 
zontally, the  petals  are  disposed  in  an  all-sided  way.  Cro- 
cuses, Tulips,  and  Poppies  are  familiar  examples  of  this  struc- 
ture occurring  under  these  conditions.  A  Ranunculus  flower, 
Fig.  228,  will  serve  as  a  typical  one.  Similarly,  flowers 
which  have  peduncles  flexible  enough  to 
let  them  hang  directly  downwards,  and 
are  not  laterally  incommoded,  are  also 
radial ;  as  in  the  Fuchsia,  Fig.  229,  as 
in  Cyclamen,  Hyacinth,  &c.  These  rela- 
tions of  form  to  position  are,  I  believe, 
uniform.  Though  some  flowers  carried  at  the  ends  of  up- 
right or  downright  stems  have  oblique  shapes,  it  is  only  when 
they  have  inclined  axes  or  are  not  equally  conditioned  all 
round.  No  solitary  flower  having  an  axis  habitually  ver- 
tical, presents  a  bilateral  form.  This  is  as  we  should  expect, 
since  flowers  which  open  out  their  faces  horizontally, 
whether  facing  upwards  or  downwards,  are,  on  the  average, 
similarly  affected  on  all  sides. 

At  first  it  seems  that  flowers  thus  placed  should  alone 
be  radial ;  but  further  consideration  discloses  conditions  under 
which  this  type  of  symmetry  may  exist  in  flowers  otherwise 
placed.  Remembering  that  the  radial  form  is  the  primitive 
form — that,  morphologically  speaking,  it  results  from  the 
contraction  into  a  whorl,  of  parts  that  are  originally  arranged 
in  the  same  spiral  succession  as  the  leaves ;  we  must  expect 


1  18  MORPHOLOGICAL   DEVELOPMENT. 

it  to  continue  wherever  there  are  no  forces  tending  to  change 
it.  What  now  must  be  the  forces  tending  to  change  itP 
They  must  be  forces  which  do  not  simply  affect  differently 
the  different  parts  of  an  individual  flower  ;  but  they  must  be 
forces  which  affect  in  like  contrasted  ways  the  homologous 
parts  of  other  individual  flowers,  both  on  the  same  plant  and 
on  surrounding  plants  of  the  same  species.  A  permanent 
modification  can  be  expected  only  in  cases  where,  by  inherit- 
ance, the  effect  of  the  modifying  causes  accumulates.  That 
it  may  accumulate  the  flowers  must  keep  themselves  so  re- 
lated to  the  environment,  that  the  homologous  parts  may 
generation  after  generation  be  subjected  to  like  differentiating 
forces.  Hence,  among  a  plant's  flowers  which  maintain  no  uni- 
formity in  the  relations  of  their  parts  to  surrounding  influences, 
the  radial  form  will  continue.  Let  us  glance  at  the  several 
causes  which  entail  this  variability.  When  flowers 

are  borne  on  many  branches,  which  have  all  inclinations  from 
the  vertical  to  the  horizontal — as  are  the  flowers  of  the  Apple, 
the  Plum,  the  Hawthorn — they  are  placed  in  countless  different 
attitudes.  Consequently,  any  spontaneous  variation  in  shape 
which  might  be  advantageous  were  the  attitude  constant,  is 
not  likely  to  be  advantageous  ;  and  any  functionally-produced 
modification  in  one  flower  is  likely  to  be  neutralized  in  off- 
spring by  some  opposite  functionally-produced  modification 
in  another  flower.  It  is  quite  comprehensible,  therefore, 
that  irregularly-branched  plants  should  thus  preserve  their 
laterally-borne  flowers  from  undergoing  permanent  devia- 
tions from  their  primitive  radial  symmetry.  Fig.  230,  re- 
***  \^  *JJf*$!&  presenting  a  blossoming 

twig  of  the  Blackthorn, 
will  illustrate  this. 
Again,  upright  panicles 
such  as  that  of  the 
Saxifrage,  shown  in  Fig. 
231,  and  irregular  terminal  groups  of  flowers  otherwise 
named,  furnish  conditions  under  which  there  is  similarly  an 


THE    SHAPES    OF    FLOWERS.  149 

absence  of  determinate  relations  between  the  parts  of  the 
flowers  and  the  incident  forces ;  and  hence  an  absence  of 
bilateralness.  This  inconstancy  of  relative  position 

is  produced  in  various  other  ways — by  extreme  flexibility  of 
the  peduncles,  as  in  the  Blue-bell ;  by  the  tendency  of  the 
peduncles  to  curl  to  a  greater  or  less  extent  in  different 
directions,  as  in  Pyrola ;  by  special  twisting  of  the  peduncles, 
differing  in  degree  in  different  individuals,  as  in  Convol- 
vulus ;  by  extreme  flexibility  of  the  petals,  as  in  Lythrum. 
Elsewhere  the  like  general  result  arises  from  a  progressive 
change  of  attitude ;  as  in  Myosotis,  the  stem  of  which  as  it 
unfolds  causes  each  flower  to  undergo  a  transition  from  an 
upward  position  of  the  mouth  to  a  lateral  position  ;  or  as  in 
most  Cruciferce,  where  the  like  effect  follows  from  an  altered 
direction  of  the  peduncle. 

There  are,  however,  certain  seemingly  anomalous  cases 
where  radial  sympathy  is  maintained  by  laterally-placed 
flowers,  which  keep  their  parts  in  relative  positions  that  are 
tolerably  constant.  The  explanation  of  these  exceptions  is 
not  manifest.  It  is  only  when  we  take  into  account  certain 
incident  actions  liable  to  be  left  unremembered,  that  we  find 
a  probable  solution.  It  will  be  most  convenient  to  postpone 
the  consideration  of  these  cases  until  we  have  reached  the 
general  rule  to  which  they  are  exceptions. 

§  234.  Transitions  varying  in  degree  from  the  radial  to- 
wards the  bilateral,  are  common  in  flowers  that  are  borne  at 
the  ends  of  branches  or  axes  which  are  inclined  in  tolerably 
constant  ways.  "We  may  see  this  in  sundry  garden  flowers 
such  as  Petunia,  or  such  as  Tydcea  and  Achimenes  shown  in 

Figs.  232  and  233.  If  these 
plants  be  examined,  it  will 
be  perceived  that  the  mode 
of  growth  makes  the  flower 
unfold  in  a  partially  one- 
sided position ;  that  its  parts  of  attachment  have  rigidity 


150  MORPHOLOGICAL    DEVELOPMENT. 

sufficient  to  prevent  this  attitude  from  being  very  much 
interfered  with ;  and  that  though  the  individual  flowers  vary 
somewhat  in  their  attitudes,  they  do  not  vary  to  the  extent  of 
neutralizing  the  differentiating  conditions — there  remains  an 
average  divergence  from  a  horizontal  unfolding  of  the  flower, 
to  account  for  its  divergence  from  radial  symmetry. 

"We  pass  insensibly  from  forms  like  these,  to  forms  having 
bilateral  symmetry  strongly  pronounced.  Some  such  forms 
occur  among  flowers  that  grow  at  the  ends  of  upright  stems  ; 
as  in  Pinguicula,  and  in  the  Yiolet  tribe.  But  this  happens 
only  where  in  successive  generations  the  flower  unfolds  its 
parts  sideways  in  constant  relative  positions.  And  in  the 
immense  majority  of  flowers  that  have  well-marked  two-sided 
forms,  the  habitual  exposure  of  the  different  parts  to  different 
sets  of  forces,  is  effectually  secured  by  the  mode  of  placing. 
As  illustrations  I  may  name  the  genera — Orchis,  Utricularifi, 
Salvia,  Salix,  Delphinum,  Mentha,  Teucrium,  Ajuga,  Battota, 
Galeopsis,  Lamium,  Stachys,  Glechoma,  Marnibiitm,  Cala- 
mintha,  Clinopodium,  Melittis,  Prunella,,  Scittellaria,  Brtrt- 
sia,  Euphrasia,  Rhinanthus,  Mclampyrum,  Pcdicularis,  Lin- 
aria,  Digitalis,  Orobanche,  Fumaria,  8fc.  /  to  which  may  be 
added,  all  the  Grasses  and  all  the  Papilionacece.  In  most  of 
these  cases  the  flowers,  being  sessile  on  the  sides  of  upright 
stems,  are  kept  in  quite  fixed  attitudes  ;  and  in  the  other 
cases  the  peduncles  are  very  short,  or  else  stiff  enough  to 
secure  general  uniformity  in  the  positions.  A  few  of  the 
more  marked  types  are  shown  in  Figs.  234  to  241. 


Very  instructive  evidences  here  meet  us.     Sometimes  with 
in  the  limits  of  one  genus  we  find  radial  flowers,  bilateral 
flowers,  and  flowers  of  intermediate  characters.     The  genus 
Brgonia  may  be  instanced.     In  B.  rigid  i  the  flowers,  various 


THE    SHAPES    OF    FLOWERS. 


151 


in  their  attitudes,  are  in  their  more  conspicuous  characters 
radial :  though  there  is  a  certain  bilateralness  in  the  calyx, 
the  five  petals  are  symmetrically  disposed  all  round.  3. 
Wagmriana  furnishes  two  forms  of  flowers  :  on  the  same  in- 
dividual plant  may  be  found  radial  flowers  like  Fig.  242,  and 
others  like  Fig.  243  that  are  merging  into  the  bilateral. 
More  decided  is  the  bilateralness  in  B.  albo-coccinea,  Fig.  244 ; 
and  still  more  in  B.  nitida,  Fig.  245.  While  in  B.  jatrophoe- 


folia,  Fig.  246,  the  change  reaches  its  extreme  by  the  dis- 
appearance of  the  lateral  petals.  On  examining  the  modes  of 
growth  in  these  several  species,  they  will  be  seen  to  explain 
these  changes  in  the  manner  alleged.  Even 

more  conclusive  are  the  nearly-allied  transformations  occur- 
ring in  artificially-produced  varieties  of  the  same  species. 
Gloxinia  may  be  named  in  illustration.  In  Fig.  247  is  repre- 
sented one  of  the  ordinary  forms,  which  shows  us  bilateralness  of 
shape  along  with  a  mode  of  growth  that  renders  the  conditions 
alike  on  the  two  sides  while  different  above  and  below.  But 
in  G.  erecta,  Fig.  248,  we 
have  the  flower  assuming  an 
upright  attitude,  and  at  the 
same  time  assuming  the  radial 
type.  This  i&  not  to  be  inter- 
preted as  a  production  of  ra- 
dial symmetry  out  of  bilateral  symmetry,  under  the  action  of 
the  appropriate  conditions.  It  is  rather  to  be  taken  as  a  case 
of  what  is  termed  "  peloria  " — a  reversion  to  the  primitive 
radial  type,  from  which  the  bilateral  modification  had  been 
derived.  The  significant  inference  to  be  drawn  from  it  is, 
that  this  primitive  radial  type  had  an  upright  attitude;  and 


152  MORPHOLOGICAL    DEVELOPMENT. 

that  the  derivation  of  a  bilateral  type  from  it,  occurred  along 
with  the  assumption  of  an  inclined  attitude. 

We  come  now  to  a  group  of  cases  above  referred  to,  in 
which  radial  symmetry  continues  to  co-exist  with  that  con- 
stant lateral  attitude  ordinarily  accompanied  by  the  two-sided 
form.  Two  examples  will  suffice  :  one  a  very  large  flower, 
the  Hollyhock,  and  the  other  a  very  small  flower,  the  Agri- 
mony. Why  does  the  radial  form  here  remain  unchanged  ? 
and  how  does  its  continuance  consist  with  the  alleged  general 
law? 

Until  quite  recently  I  have  been  unable  to  find  any  pro- 
bable answers  to  these  questions.  When  the  difficulty  first 
presented  itself,  I  could  think  of  no  other  possible  cause  for 
the  anomaly,  than  that  the  parts  of  the  Hollyhock-flower, 
unfolding  spirally  as  they  do,  might  have  different  degrees  of 
spiral  twist  in  different  flowers,  and  might  thus  not  be  unfolded 
in  sufficiently-constant  positions.  But  this  seemed  a  very 
questionable  interpretation ;  and  one  which  did  not  obviously 
apply  to  the  case  of  the  Agrimony.  It  was  only  on  inquiring 
what  are  the  special  causes  of  modifications  in  the  forms  of 
flowers,  that  a  more  feasible  explanation  suggested  itself ;  and 
this  would  probably  never  have  suggested  itself,  had  not  Mr 
Darwin's  investigations  into  the  fertilization  of  Orchids  led 
me  to  take  into  account  an  unnoticed  agency. 

The  actions  which  affect  the  forms  of  leaves,  affect  much 
less  decidedly  the  forms  of  flowers ;  and  the  forms  of  flowers 
are  influenced  by  actions  that  do  not  influence  the  forms  of 
leaves.  Partly  through  the  direct  action  of  incident  forces 
and  partly  through  the  indirect  action  of  natural  selection, 
leaves  get  their  parts  distributed  in  ways  that  most  facilitate 
their  assimilative  functions,  under  the  circumstances  in  which 
they  are  placed ;  and  their  several  types  of  symmetry  are  thus 
explicable.  But  in  flowers,  the  petals  and  fructifying  organs 
of  which  do  not  contain  chlorophyll,  the  tendency  to  grow 
most  where  the  supply  of  light  is  greatest,  is  less  decided,  if 
not  absent ;  and  a  shape  otherwise  determined  is  hence  less 


Iltli   SHAPES   OF    FLOWERS.  153 

liable  to  alter  in  consequence  of  altered  relations  to  sun  and 
air.  Gravity,  too,  must  be  comparatively  ineffective  in  caus- 
ing modifications :  the  smaller  sizes  of  the  parts,  as  well  as 
their  modes  of  attachment,  giving  them  greater  relative 
rigidity.  Not,  indeed,  that  these  incident  forces  of  the  inor- 
ganic world  are  here  quite  inoperative.  Fig.  **9  \^> 
249,  representing  a  species  of  Campanula,  I  Y 
shows  that  the  developments  of  individual  flow-  (\H/ 
ers  are  somewhat  modified  by  the  relations  of 
their  parts  to  general  conditions.  But  the 
fact  to  be  observed  is,  that  the  extreme  trans- 
formations which  flowers  undergo  are  not 
likely  to  be  thus  caused  :  some  further  cause 
must  be  sought.  And  if  we  bear  in  mind 
the  functions  of  flowers,  we  shall  find  in  their 
adaptations  to  their  functions,  under  conditions  that  are 
extremely  varied,  an  adequate  cause  for  the  different  types 
of  symmetry,  as  well  as  for  the  exceptions  to  them.  Flow- 
ers are  parts  in  which  fertilization  is  effected;  and  the 
active  agents  of  this  fertilization  are  insects — bees,  moths, 
butterflies,  &c.  Mr  Darwin  has  shown  in  many  cases,  that 
the  forms  and  positions  of  the  essential  organs  of  fructifica- 
tion, are  such  as  to  facilitate  the  actions  of  insects  in  trans- 
ferring pollen  from  the  anthers  of  one  flower  to  the  pistil  of 
another  —  an  arrangement  produced  by  natural  selection. 
And  here  we  shall  find  reason  for  concluding,  that  the  forms 
and  positions  of  those  subsidiary  parts  which  give  the  gene- 
ral shape  to  the  flower,  similarly  arise  by  the  survival  of 
individuals  which  have  the  subsidiary  parts  so  adjusted  as  to 
aid  this  fertilizing  process — the  deviations  from  radial  sym- 
metry being  among  such  adjustments.  The  reasoning  is  as 
follows.  So  long  as  the  axis  of  a  flower  is  vertical  and 

the  conditions  are  similar  all  round,  a  bee  or  butterfly  alight- 
ing on  it,  will  be  as  likely  to  come  from  one  side  as  from 
another ;  and  hence,  hindrance  rather  than  facilitation  would 
result  if  the  several  sides  of  the  flower  did  not  afford  it  equally 


154  MORPHOLOGICAL   DEVELOPMENT. 

free  access.  In  like  manner,  flowers  which  are  distributed 
over  a  plant  in  such  ways  that  their  discs  open  out  on 
planes  of  all  directions  and  inclinations,  will  have  no  tend- 
ency to  lose  their  radial  symmetry ;  since,  on  the  average, 
no  part  of  the  periphery  is  differently  related  to  insect- 
agency  from  any  other  part.  But  flowers  so  fixed  as  to 
open  out  sideways  in  tolerably-constant  attitudes,  have 
their  petals  differently  related  to  insect-agency.  A  bee  or 
butterfly  coming  to  a  laterally- growing  flower,  does  not  set- 
tle on  it  in  one  way  as  readily  as  in  another ;  but  almost  of 
necessity  settles  with  the  axis  of  its  body  inclined  upwards 
towards  the  stem  of  the  plant.  Hence,  the  side-petals  of  a 
flower  so  fixed,  habitually  stand  to  the  alighting  insect  in 
relations  different  from  those  in  which  the  upper  and  lower 
petals  stand ;  and  the  upper  and  lower  petals  differ  from  one 
another  in  their  relations  to  it.  If,  then,  there  so  arises  an 
habitual  attitude  of  the  insect  towards  the  petals,  there  must 
be  some  particular  arrangement  of  the  petals  that  will  be 
most  convenient  to  the  insect — will  most  facilitate  its  en- 
trance into  the  flower.  Thus  we  see  in  many  cases,  that  a 
long  undermost  petal  or  lip,  by  enabling  the  insect  to  settle 
in  such  way  as  to  bring  its  head  opposite  to  the  opening  of 
the  tube,  aids  its  fertilizing  agency.  But  whatever  be  the  spe- 
cial modifications  of  the  corolla  which  facilitate  the  actions  of 
the  particular  insects  concerned,  all  of  them  will  conduce  to 
bilateral  symmetry ;  since  they  will  be  alike  for  the  two  sides 
but  unlike  for  the  top  and  bottom.  And  now  we 

are  prepared  for  understanding  the  exceptions.  Flowers 
growing  sideways  can  become  thus  adapted  by  survival  of 
the  fittest,  only  if  they  are  of  such  sizes  and  structures  that 
insect-agency  can  affect  them  in  the  way  described.  But 
in  the  plants  named  above,  this  condition  is  not  fulfilled.  A 
Hollyhock-flower  is  so  open,  as  well  as  so  large,  that  its  petals 
are  not  in  any  appreciable  degree  differently  related  to  the 
insects  which  visit  it.  On  the  other  hand,  the  flower  of  the 
Agrimony  is  so  small,  that  unless  visited  by  insects  of  a 


THE    SHAPES   OF    FLOWERS.  155 

corresponding  size  which  settle  as  bees  and  butterflies  settle, 
its  parts  will  not  be  affected  in  the  alleged  manner.  That 
all  anomalies  of  this  kind  can  at  once  be  satisfactorily  ex- 
plained, is  scarcely  to  be  expected :  the  circumstances  of 
each  case  have  to  be  studied.  But  it  seems  not  improbable 
that  they  are  all  due  to  causes  of  the  kind  indicated. 

§  235.  "We  have  already  glanced  at  clusters  of  flowers 
for  the  purpose  of  considering  their  shapes  as  clusters.  We 
must  now  return  to  them  to  observe  the  modifications  under- 
gone by  their  component  flowers.  Among  these  occur  illus- 
trations of  great  significance. 

An  example  of  transition  from  the  radial  to  the  bilateral 
form  in  clustered  flowers  of  the  same  species,  is  furnished  by 
the  cultivated  Geraniums,  called  by  florists  Pelargoniums. 
Some  of  these  bearing  somewhat  small  terminal  clusters 
of  flowers,  which  are  closely  packed  together,  with  their 
faces  almost  upwards,  have  radially-symmetrical  flowers. 
But  among  other  varieties  having  terminal  clusters  of  which 
the  members  are  mutually  thrust  on  one  side  by  crowding, 
the  flowers  depart  very  considerably  from  the  radial  shape 
towards  the  bilateral  shape.  A  like  result  occurs  under 
like  conditions  in  Rhododendrons  and  Azaleas.  The  Verbena, 
too,  furnishes  an  illustration  of  radial  flowers  rendered 
slightly  two-sided  by  the  slight  two-sideness  of  their  rela- 
tions to  other  flowers  in  the  cluster.  And  among  the  Cruci- 
ferce,  a  kindred  case  occurs  in  the  cultivated  Candytuft. 

Evidence  of  a  somewhat  different  kind,  is  offered  us  by 
clustered  flowers  in  which  the  peripheral  members  of  the 
clusters  differ  from  the  central  members ;  and  this  evidence 
is  especially  conclusive  where  we  find  allied  species  that  do 
not  exhibit  the  deviation,  at  the  same  time  that  they  do  not 
fulfil  the  conditions  under  which  it  may  be  expected.  Thus, 
in  Scabiosa  succisa,  Fig.  250,  which  bears  its  numerous  small 
flowers  in  a  hemispherical  knob,  the  component  flowers, 
similarly  circumstanced,  are  all  equal  and  all  radial ;  but  in 


I5C  MORPHOLOGICAL    DEVELOPMENT. 

Scabiosa  arvensis,  Fig.  251,  in  which  the  numerous  small 
flowers  form  a  flattened  disk, 
^  only  the  confined  central  ones 
3£  are  radial :  round  the  edge  the 
flowers  are  much  larger,  and 
conspicuously  bilateral. 

But  the  most  remarkable 
and  most  conclusive  proofs  of  these  relations  between  forms 
and  positions,  are  those  given  by  the  clustered  flowers  called 
Umbelliferce.  In  some  cases,  as  where  the  component  flowers 
have  all  plenty  of  room,  or  where  the  surface  of  the  umbel  is 
more  or  less  globular,  the  modifications  are  not  conspicuous ; 
but  where,  as  in  Viburnum,  Chceroj)hyllum,  Anthriscits,  Torilis, 
Caucalis,  Daucus,  Tordylium,  &c.,  we  have  flowers  clustered 
in  such  ways  as  to  be  differently  conditioned,  we  find  a  num- 
ber of  modifications  that  are  marked  and  varied  in  propor- 
tion as  the  differences  of  conditions  are  marked  and  varied. 
In  Chcerophyllum,  where  the  flowers  of  each  umbellule  are 
closely  placed  so  as  to  form  a  flat  surface,  but  where  the 
umbellules  are  wide  apart  and  form  a  dispersed  umbel,  the 
umbellules  do  not  differ  from  one  another ;  though  among  the 
flowers  of  each  umbellule  there  are  decided  differences — the 
central  flowers  being  small  and  radial,  while  the  peripheral 
ones  are  large  and  bilateral.  But  in  other  genera,  where  not 
only  the  flowers  of  each  umbellule  but  also  the  umbellules 
themselves  are  closely  clustered  into  a  flat  surface,  the  umbel- 
lules themselves  become  contrasted;  and  many  remarkable 
secondary  modifications  arise.  In  an  umbel  of  Heracleum, 
for  instance,  there  are  to  be  noted  the  facts : — first,  that  the 
external  umbellules  are  larger  than  the  internal  ones ; 
second,  that  in  each  umbellule  the  central  flowers  are  less 
developed  than  the  peripheral  ones  ;  third,  that  this  greater 
development  of  the  peripheral  flowers  is  most  marked  in  the 
outer  umbellules ;  fourth,  that  it  is  most  marked  on  the  outer 
sides  of  the  outer  umbellules ;  fifth,  that  while  the  interior 
flowers  of  each  umbellule  are  radial,  the  exterior  ones  are 


THE    SHAPES    OF    FLOWERS. 


157 


bilateral ;  sixth,  that  this  bilateralness  is  most  marked  in 
the  peripheral  flowers  of  the  peripheral  umbellules ;  seventh, 
that  the  flowers  on  the  outer  side  of  these  peripheral 
umbellules  are  those  in  which  the  bilateralness  reaches  a 
maximum ;  and  eighth,  that  where  the  outer  umbellules 
touch  each  other,  the  flowers,  being  unsymmetrically 
placed,  are  unsymmetrically  bilateral.*  The  like  modi- 
fications are  displayed,  though  not  in  so  clearly-trace- 
able a  way,  in  an  umbel  of  Tordylium,  Fig.  252.  Considering 
how  obviously  these  various 
forms  are  .related  to  the  vari- 
ous conditions,  we  should  be 
scarcely  able,  even  in  the 
absence  of  all  other  facts,  to 
resist  the  conclusion  that  the 
differences  in  the  conditions 
are  the  causes  of  the  differ- 
ences in  the  forms. 

Composite  flowers  furnish 
evidence  so  nearly  allied  to 
that  which  clustered  flowers 
furnish,  that  we  may  fitly  glance  at  them  under  the  same 
head.  Such  a  common  type  of  this  order  as  the  Sun- flower, 
exemplifies  the  extremely  marked  differ- 
ence that  arises  in  many  of  these  plants 
between  the  closely-packed  internal 
florets,  each  similarly  circumstanced  on 
all  sides,  and  the  external  florets,  not 
similarly  circumstanced  on  all  sides. 
In  Fig.  253,  representing  the  inner  and 
outer  florets  of  a  Daisy,  the  contrast  is 
marked  between  the  small  radial  corolla  of  the  one  and  the 
larger  bilateral  corolla  of  the  other.  In  many  cases,  how- 
ever, this  contrast  is  less  marked :  the  inner  florets  having 

*  I  had  intended  here  to  insert  a  figure  exhibiting  these  differences ;  but  as  the 
Cow-parsnip  does  not  flower  till  July,  and  as  I  can  find  no  drawing  of  the  umbel 
which  adequately  represents  its  details,  I  am  obliged  to  take  another  instance. 


353 


158  MORPHOLOGICAL    DEVELOPMENT. 

also  their  outward-growing  prolongations — a  difference  pos- 
sibly related  to  some  difference  in  the  habits  of  the  insects 
that  fertilize  them.  Nevertheless,  these  composite  flowers 
which  have  inner  florets  with  strap-shaped  corollas  out- 
wardly directed,  equally  conform  to  the  general  principle ; 
both  in  the  radial  arrangement  of  the  assemblage  of  florets, 
and  in  the  bilateral  shape  of  each  floret ;  which  has  its 
parts  alike  on  the  two  sides  of  a  line  passing  from  the  centre 
of  the  assemblage  to  the  circumference.  '  Certain 

other  members  of  this  order  fulfil  the  law  somewhat  differ- 
ently. In  Centaurea,  for  instance,  the  inner  florets  are  small 
and  vertical  in  direction,  while  the  outer  florets  are  large  and 
lateral  in  direction.  And  here  may  be  remarked,  in  passing, 
a  clear  indication  of  the  effect  which  great  flexibility  of  the 
petals  has  in  preventing  a  flower  from  losing  its  original 
radiate  form  ;  for  while  in  C.  cyanus,  the  large  outward- grow- 
ing florets,  having  short,  stiff  divisions  of  the  corolla,  are 
decidedly  bilateral,  in  C.  scabiosa,  where  the  divisions  of  the 
corolla  are  long  and  flexible,  the  radial  form  is  scarcely  at 
all  modified.  On  bearing  in  mind  the  probable  relations  of 
the  forms  to  insect-agency,  the  meaning  of  this  difference 
will  not  be  difficult  to  understand. 

§  236.  In  extremely- varied  ways  there  are  thus  re-illus- 
trated among  flowers,  the  general  laws  of  form  which  leaves 
and  branches  and  entire  plants  disclose  to  us.  Composed  as 
each  cluster  of  flowers  is  of  individuals  that  are  originally 
similar ;  and  composed  as  each  flower  is  of  homologous  foliar 
organs ;  we  see  both  that  the  like  flowers  become  unlike  and 
the  like  parts  of  each  flower  become  unlike,  where  the  posi- 
tions involve  unlike  incidence  of  forces.  The  symmetry 
remains  radial  where  the  conditions  are  equal  all  round  ; 
shows  deviation  towards  two-sidedness  where  there  is  slight 
two-sidedness  of  conditions ;  becomes  decidedly  bilateral 
where  the  conditions  are  decidedly  bilateral ;  and  passes  into 
an  unsymmetrical  form  where  the  relations  to  the  environ- 
ment are  unsymmetrical. 


CHAPTER  XI. 

TKE    SHAPES   OF   VEGETAL   CELLS. 

§  237.  WE  come  now  to  aggregates  of  the  lowest  order. 
Already  something  has  been  said  (§  217)  concerning  the 
forms  of  those  morphological  units  which  exist  as  independent 
plants.  But  it  is  here  requisite  briefly  to  note  the  modifica- 
tions undergone  by  them  where  they  become  components  of 
larger  plants. 

Of  the  numerous  cell- forms  which  are  found  in  the  tissues 
of  the  higher  plants,  it  will  suffice  to  give,  in  Fig.  254,  re- 
presenting a  section  of  the  surface  of 
a  leaf,  a  single  example.  In  this  it 
will  be  seen  that  the  epidermis  cells 
c,  covered  by  the  secreted  external 
layer  a,  and  separated  from  the  layer 
of  cells  below  them  by  the  masses  of 
inter- cellular  substance  b,  have  differ- 
entiations of  form  clearly  related  to 
differences  in  the  incidence  of  forces.  Their  divergences  from 
primordial  sphericity  are  such  as  correspond  with  the  un- 
likenesses  in  the  circumstances  of  their  respective  sides. 
Similarly  with  the  layers  below  them.  And  throughout  the 
more  complex  modifications  which  the  cells  of  other  tissues 
exhibit,  the  like  correspondence  holds. 

Among  plants  of  a  lower  order  of  aggregation,  we  hart  al- 
ready seen  how  cells  become  metamorphosed  as  they  become 
integrated  into  masses  having  definite  organizations.  The 


160 


MORPHOLOGICAL    DEVELOPMENT. 


higher  Algce,  exemplified  in  Figs.  32,  34,  35,  show  this  very 
clearly.  Here  the  departure  from 
the  simple  cell-form  to  the  form  of 
an  elongated  prism,  is  manifestly 
t  subordinated  to  the  contrasts  in  the 
relations  of  the  parts.  And  it  is 
interesting  to  observe  how,  in  one 
of  the  branches  of  Fig.  32,  we  pass 
from  the  small,  almost-spherical 
cells  which  terminate  the  branch- 
lets,  to  the  large,  much-modified 
cells  which  join  the  main  stem, 

through    gradations    obviously   related    in    their   changed 

forms  to  the  altered  actions  their  positions  expose  them  to. 
More  simply,  but  quite  as  conclusively,  do  the  inferior 

Algte,  of  which  Figs.  19 — 23  are  examples,  show  us  how 


cells  pass  from  their  original  spherical  symmetry  into  radial 
symmetry,  as  they  pass  from  a  state  in  which  they  are  simi- 
larly-conditioned on  all  sides,  to  a  state  in  which  two  of  their 
opposite  sides  or  ends  are  conditioned  in  ways  that  are  like 
one  another,  but  unlike  the  ways  in  which  all  other  sides  are 
conditioned. 

Still  more  instructive  are  the  morphological  differentiations 
of  those  protophytes  in  which  the  first  steps  towards  a  higher 
degree  of  integration  are  shown.  Fig.  9  represents  one  of 
the  transitional  forms  of  Dcsmidiacece.  In  it  we  see  that  the 
two  inner  halves  by  which  the  individuals  are  united,  differ 


THE    SHAPES   OF    VEGETAL    CELLS. 


1G1 


somewhat  from  the  two  outer  halves.  So,  too,  of  the  type 
exemplified  by  Fig.  10,  it  is  to  be  noted  that  besides  the 
difference  between  the  transverse  and  longitudinal  dimensions, 
which  the  component  units  display  in  common,  the  two  end 
units  differ  from  the  rest :  they  have  appendages  which  the 


rest  have  not.  Once  more,  where  the  integration  is  car- 
ried on  in  such  ways  as  to  produce  not  strings  but  clusters, 
there  arise  contrasts  and  correspondences  just  such  as  might 
be  looked  for.  All  the  four  members  of  the  group  shown  in 
Fig.  12,  are  similarly  conditioned ;  and  each  of  them  has 
a  bilateral  shape  answering  to  its  bilateral  relations.  In 
Fig.  14  we  have  a  number  of  similarly- bilateral  individuals 
on  the  circumference,  including  a  central  individual  differing 
from  the  rest  by  having  the  bilateral  character  nearly 
obliterated.  And  then,  in  Fig.  15,  we  have  two  central 
components  of  the  group,  deviating  more  decidedly  from 
those  that  surround  them. 

These  few  typical  facts,  which  must  be  taken  like  the  few 
typical  facts  grouped  in  each  of  the  foregoing  chapters  as 
indicating  a  mass  of  evidence  too  great  to  be  here  detailed, 
will  sufficiently  show  that  froui  the  most  complex  vegetal 
types  down  to  the  most  simple,  the  laws  of  morphological 
differentiation  remain  the  same. 


CHAPTER  XII. 


CHANGES   OF    SHAPE   OTHERWISE   CAUSED. 

§  238.  BESIDES  the  more  special  causes  of  modification  in 
the  shapes  of  plants  and  of  their  parts,  certain  more  general 
causes  must  be  briefly  noticed.  These  may  be  described  as 
consequences  of  variations  in  the  total  quantities  of  the 
matters  and  forces  furnished  to  plants  by  their  environments. 
Some  of  the  changes  of  form  so  produced  are  displayed  by 
plants  as  wholes,  and  others  only  by  their  parts.  "We  will 
glance  at  them  in  this  order. 

§  239.  It  is  a  familiar  fact  that  luxuriant  shoots  have  re- 
latively-long internodes ;  and,  conversely,  that  a  shoot 
dwarfed  from  lack  of  sap,  has  its  nodes  closely  clustered  :  the 
result  being  that  the  lateral  axes,  where  these  are  developed 
become  in  the  one  case  far  apart  and  in  the  other  case  neai 
together.  Fig.  255  represents  a  branch  to  the  parts  of  which 
the  longer  and  shorter  internodes  so  result- 
ing  give  differential  characters.  A  whole 
tree  being  in  many  cases  simultaneously 
thus  affected  by  states  of  the  earth  or  the 
air,  all  parts  of  it  may  have  such  varia- 
tions  impressed  on  them;  and,  indeed,  such 
variations,  following  more  or  less  regu- 
larly  the  changes  of  the  seasons,  give  to 
many  trees  manifest  traits  of  structure. 
In  Fig.  256,  a  shoot  of  Phyllocactua 


CHANGES    OF    SHAPE    OTHERWISE    CAUSED.  163 

crenatus,  we  have  an  interesting  example  of  a  variation  essen- 
tially of  the  same  nature,  little  as  it  appears  to  be  so.  For 
each  of  the  lateral  indentations  is  here  the  seat  of  an  axillary 
bud ;  and  these  we  see  are  separated  by  internodes  which, 
becoming  broader  as  they  become  longer,  and  narrower  as 
they  become  shorter,  produce  changes  of  form  that  correspond 
with  changes  in  the  luxuriance  of  growth. 

To  complete  the  statement  it  must  be  added  that  these 
variations  of  nutrition  often  determine  the  development  or 
non-development  of  lateral  axes  ;  and  by  so  doing  cause  still 
more  marked  structural  differences.  The  Fox-glove  may  be 
named  as  a  plant  which  illustrates  this  truth. 

§  240.  From  the  morphological  differentiations  caused  by 
unlikeiiesses  of  nutrition  which  the  whole  plant  feels,  we  pass 
now  to  those  which  are  thus  caused  in  some  of  its  parts  and 
not  in  others.  Among  such  are  the  contrasts  between 
flowering  axes,  and  the  axes  that  bear  leaves  only.  It  has 
already  been  shown  in  §  78,  that  the  belief  expressed  by 
Wolff  in  a  direct  connexion  between  fructification  and  innu- 
trition, is  justified  inductively  by  many  facts  of  many  kinds. 
Deductively  too,  in  §  79,  we  saw  reason  to  conclude  that  such 
a  relation  would  be  established  by  survival  of  the  fittest ; 
seeing  that  it  would  profit  a  species  for  its  members  to  begin 
sending  off  migrating  germs  from  the  ends  of  those  axes 
which  innutrition  prevented  from  further  agamogenetic  mul- 
tiplication. Once  more,  when  considering  the  nature  of  the 
phsenogamic  axis,  we  found  support  for  this  belief  in  the  fact 
that  the  components  of  a  flower  exhibit  a  reversion  to  that 
type  from  which  the  phaenogamic  type  has  probably  arisen — 
a  reversion  which  the  laws  of  embryology  would  lead  us  to 
look  for  where  innutrition  had  arrested  development. 

Hence,  then,  we  may  properly  count  those  deviations  of 
structure  which  constitute  inflorescence,  as  among  the  mor- 
phological differentiations  produced  by  local  innutrition.  I  do 
not  mean  that  the  detailed  modifications  which  the  essential 


164  MORPHOLOGICAL   DEVELOPMENT 

and  subservient  organs  of  fructification  display,  are  thus 
accounted  for :  we  have  seen  reason  to  think  them  otherwise 
caused.  But  I  mean  that  the  morphological  characters  which 
distinguish  gamogenetic  axes  in  general  from  agamogenetic 
axes,  such  as  non-development  of  the  internodes,  and  dwarf- 
ing of  the  foliar  organs,  are  primarily  results  of  failure  in 
tho  supply  of  some  material  required  for  further  growth.* 

§  241.  Another  trait  which  has  to  be  noticed  under  this 
head,  is  the  spiral,  or  rather  the  helical,  arrangement  of 
parts.  The  successive  nodes  of  a  phgenogam  habitually  bear 
their  appendages  in  ways  implying  more  or  less  twist  in  the 
substance  of  the  axis ;  and  in  climbing  plants  the  twist  is  such 
as  to  produce  a  corkscrew  shape.  This  structure  is  ascribable 
to  differences  of  interstitial  nutrition.  Taking  a  shoot  that 
is  growing  vertically,  it  is  clear  that  if  the  molecules  are 
added  with  perfect  equality  on  all  sides,  there  will  be  no 
tendency  towards  any  kind  of  lateral  deviation  ;  and  the 
successively-produced  parts  will  be  perpendicularly  over  one 

*  It  is  but  just  to  the  memory  of  "Wolff,  here  to  point  out  that  he  was  im- 
mensely in  advance  of  Goethe  in  his  rationale  of  these  metamorphoses.  Whatever 
greater  elaboration  Goethe  gave  to  the  theory  considered  as  an  induction,  seems 
to  me  more  than  counter-balanced  by  the  irrationality  of  his  deductive  interpret- 
ation ;  which  unites  mediaeval  physiology  with  Platonic  philosophy.  A  domin- 
ant idea  with  him  is  that  leaves  exist  for  the  purpose  of  carrying  off  crude  juices— 
that  "as  long  as  there  are  crude  juices  to  be  carried  off,  the  plant  must  be  pro- 
vided with  organs  competent  to  effect  the  task ;"  that  while  "  the  less  pure  fluids 
are  got  rid  of,  purer  ones  are  introduced ;"  and  that "  if  nourishment  is  withheld, 
that  operation  of  nature  (flowering)  is  facilitated  and  hastened ;  the  organs  of  the 
nodes  (leaves)  become  more  refined  in  texture,  the  action  of  the  purified  juices 
becomes  stronger,  and  the  transformation  of  parts  having  now  become  possible, 
takes  place  without  delay."  This  being  the  proximate  explanation,  the  ultimate 
explanation  is,  that  Nature  wishes  to  form  flowers — that  when  a  plant  flowers  it 
"attains  the  end  prescribed  to  it  by  nature  ;"  and  that  so  "  nature  at  length  at- 
tains her  object."  Instead  of  vitiating  his  induction  by  a  teleology  that  is  as 
unwarranted  in  its  assigned  object  as  in  its  assigned  means,  Wolff  ascribes  the 
phenomena  to  a  cause  which,  whether  sufficient  or  not,  is  strictly  scientific  in 
its  character.  Variation  of  nutrition  is  unquestionably  a  "  true  cause  "  of  vari- 
ation in  plant-structure.  We  have  here  no  imaginary  action  of  a  fictitious  agency  ; 
but  an  ascertained  action  of  a  known  agency. 


CHANGES    OF   SHAPE    OTHERWISE    CAuSED.  165 

another.  But  any  inequality  in  the  rate  of  growth-on  the 
different  sides  of  the  shoot,  will  destroy  this  straightness  in 
the  lines  of  growth.  If  the  greatest  and  least  rates  of  mole- 
cular increase  happens  to  be  on  opposite  sides,  the  shoot  must 
assume  a  curve  of  single  curvature  ;  but  in  every  other  case 
of  unequal  molecular  increase,  a  curve  of  double  curvature 
will  result.  Now  it  is  a  corollary  from  the  instability  of  the 
homogeneous,  that  the  rates  of  growth  on  all  sides  of  a  shoot 
can  never  be  exactly  alike  ;  and  it  is  to  be  also  inferred  from 
the  same  general  law,  that  the  greatest  and  least  rates  of  growth 
will  not  occur  on  exactly  opposite  sides  of  the  shoot,  at  the 
same  time  that  equal  rates  of  growth  are  preserved  by  the 
two  other  sides.  Hence,  there  must  almost  inevitably  arise 
more  or  less  of  twist ;  and  the  appendages  of  the  internodes 
will  so  be  prevented  from  occurring  perpendicularly  one  over 
another. 

A  deviation  of  this  kind,  necessarily  initiated  by  physical 
causes  in  conformity  with  the  general  laws  of  evolution,  is 
likely  to  be  made  regular  and  decided  by  natural  selection. 
For  under  ordinary  circumstances,  a  plant  will  profit  by  hav- 
ing its  axis  so  twisted  as  to  bring  the  appended  leaves  into 
positions  that  prevent  them  from  shading  one  another.  And, 
manifestly,  modifications  in  the  forms,  sizes,  and  insertions 
of  the  leaves,  may,  under  the  same  agency,  lead  to  adapted 
modifications  of  the  twist.  We  must  therefore  ascribe  this 
common  characteristic  of  phaenogams,  primarily  to  local  differ- 
ences of  nutrition,  and  secondarily  to  survival  of  the  fittest. 

It  is  proper  to  add  that  there  are  some  Monocotyledons, 
as  Urania  speciosa,  in  which  this  character  does  not  occur. 
What  conditions  of  existence  they  are  that  here  hold  this 
natural  tendency  in  check,  it  is  not  easy  to  see.* 

*  The  Natural  History  Review  for  July,  1865,  contained  an  article  on  the  doc. 
trine  of  morphological  composition  set  forth  in  the  foregoing  Chaps.  I.  to  III.  In 
this  article,  which  unites  exposition  and  criticism  in  a  way  that  is  unhappily  not 
common  with  reviewers,  it  is  suggested  that  the  spiral  structure  may  he  caused 
hy  natural  selection.  When  this  article  appeared,  the  foregoing  five  pages  were 
Bt.anding  over  in  type,  as  surplus  from  No.  14,  issued  in  June,  1865. 


CHAPTER  XIII. 

MORPHOLOGICAL   DIFFERENTIATION   IN  ANIMALS. 

§  242.  THE  general  considerations  which  preluded  our  in- 
quiry into  the  shapes  of  plants  and  their  parts,  equally  serve, 
so  far  as  they  go,  to  prelude  an  inquiry  into  the  shapes  of 
animals  and  their  parts.  Among  animals,  as  among  plants, 
the  formation  of  aggregates  greater  in  bulk  or  higher  in 
degree  of  composition,  or  both,  is  accompanied  by  changes  of 
form  in  the  aggregates  as  wholes  as  well  as  by  changes  of 
form  in  their  parts  ;  and  the  processes  of  morphological 
differentiation  conform  to  the  same  general  laws  in  the  one 
kingdom  as  in  the  other. 

It  is  needless  to  recapitulate  the  several  kinds  of  modifi- 
cation to  be  explained,  and  the  several  factors  that  co- 
operate in  working  them.  In  so  far  as  these  are  common 
to  plants  and  animals,  the  preceding  chapters  have  suf- 
ficiently familiarized  them.  Nor  is  it  needful  to  specify 
afresh  the  several  types  of  symmetry  and  their  descriptive 
names  ;  for  what  is  true  of  them  in  the  one  case  is  true  of 
them  in  the  other.  There  is,  however,  one  new  and  all- 
important  factor  which  we  shall  have  now  to  take  into 
account ;  and  about  this  a  few  preliminary  remarks  are 
requisite. 

§  243.  This  new  factor  is  motion — motion  of  the  organism 
in  relation  to  surrounding  objects,  or  of  the  parts  of  the 


MORPHOLOGICAL    DIFFERENTIATION    IN    ANIMALS.          167 

organism  in  relation  to  one  another,  or  both.  Though  there 
are  plants,  especially  of  the  simpler  kinds,  which  move, 
and  though  a  few  of  the  simpler  animals  do  not  move  ;  yet 
movements  are  so  exceptional  and  unobtrusive  in  the  one 
kingdom,  while  they  are  so  general  and  conspicuous  in  the 
other,  that  the  broad  distinction  commonly  made  is  well 
warranted.  What,  among  plants,  is  an  inappreciable  cause 
of  morphological  differentiation,  becomes,  among  animals,  the 
chief  cause  of  morphological  differentiation. 

Animals  that  are  rooted  or  otherwise  fixed,  of  course  present 
traits  of  structure  nearest  akin  to  those  we  have  been  latelv 
studying.  The  motions  of  parts  in  relation  to  one  another 
and  to  the  environment,  being  governed  by  thp.  mode  of  aggre- 
gation and  mode  of  fixing,  we  are  presented  with  morphological 
differentiations  similar  in  their  general  characters  to  those  of 
plants,  and  showing  us  parallel  kinds  of  symmetry  under 
parallel  conditions.  But  animals  which  move  from  place 
to  place  are  subject  to  an  additional  class  of  actions  and  re- 
actions. These  actions  and  reactions  affect  them  in  various 
ways  according  to  their  various  modes  of  movement.  Let  us 
glance  at  the  several  leading  relations  between  shape  and 
motion  which  we  may  expect  to  find. 

If  an  organism  advances  through  a  homogeneous  medium 
with  one  end  always  foremost,  that  end,  being  exposed  to 
forces  unlike  those  to  which  the  other  end  is  exposed,  may 
be  expected  to  become  unlike  it ;  and  supposing  this  to  be 
the  only  constant  contrast  of  conditions,  we  may  expect  an 
equal  distribution  of  the  parts  round  the  axis  of  move- 
ment— a  radial  symmetry.  If  in  addition  to  this 
habitual  attitude  of  the  ends,  one  surface  of  the  body  is 
always  uppermost  and  another  always  lowermost,  there  arise 
between  the  top  and  bottom  dissimilarities  of  conditions, 
while  the  two  sides  remain  similarly  conditioned.  Hence  it 
is  inferable  that  such  an  organism  will  be  divisible  into 
similar  halves  by  a  vertical  plane  passing  through  its  axis  of 
motion — will  have  a  bilateral  symmetry.  We  may  presume 
VOL.  II.  8 


168  MORPHOLOGICAL    DEVELOPMENT. 

that  this  symmetry  will  deviate  but  little  from  double 
bilateralness  where  the  upper  and  under  parts  are  not  exposed 
to  strongly-contrasted  influences ;  while  we  may  rationally 
look  for  single  bilateral  symmetry  of  a  decided  kind,  in 
creatures  having  dorsal  and  ventral  parts  conversant  witli 
very  unlike  regions  of  the  environment :  as  in  all  cases 
where  the  movement  is  over  a  solid  surface.  If  the 

movement,  though  over  a  solid  surface,  is  not  constant  in 
direction,  but  takes  place  as  often  on  one  side  as  on  another, 
radial  symmetry  may  be  again  looked  for ;  and  if  the  motions 
are  still  more  variously  directed — if  they  are  not  limited  to 
approximately-plane  surfaces,  but  extend  to  surfaces  that  are 
distributed  all  around  with  a  regular  irregularity — an  ap- 
proach of  the  radial  towards  the  spherical  symmetry  is  to  be 
anticipated.  Where  the  habits  are  such  that  the 

intercourse  between  the  organism  and  its  environment,  does 
not  involve  an  average  equality  of  actions  and  reactions  on 
any  two  or  more  sides,  there  may  be  expected  either  total 
irregularity  or  some  divergence  from  regularity. 

The  like  general  relations  between  forms  and  incident 
forces  are  inferable  in  the  component  parts  of  animals,  as 
well  as  in  the  animals  as  wholes.  It  is  needless,  however,  to 
occupy  space  by  descriptions  of  these.  Let  us  now  pass  to 
the  facts,  and  see  how  they  confirm,  a  posteriori,  the  con- 
clusions here  reached  a  priori. 


CHAPTER  XIV. 

THE  GENERAL  SHAPES  OF  ANIMALS. 

§  244.  CERTAIN  of  the  Protozoa  are  quite  indefinite  in 
their  shapes,  and  quite  inconstant  in  those  indefinite  shapes 
which  they  have — the  relations  of  their  parts  are  indeter- 
minate both  .in  space  and  time.  In  one  of  the  simpler 
Rhizopods,  at  least  during  the  active  stage  of  its  existence, 
no  permanent  distinction  of  inside  and  outside  is  established ; 
and  hence  there  can  arise  no  established  correspondence 
between  the  shape  of  the  outside  and  the  distribution  of 
environing  actions.  But  when  the  relation  of  inner  and 
outer  becomes  fixed,  either  over  part  of  the  mass  or  over  the 
whole  of  it,  we  have  kinds  of  symmetry  that  correspond 
with  the  habitual  incidence  of  forces.  An  Amoeba  in  be- 
coming encysted,  which  we  may  regard  as  the  production 
in  it  of  a  differentiation  between  superficial  parts  and  central 
parts,  passes  from  an  indefinite,  ever-changing  form  into 
a  spnerical  form  ;  and  the  order  of  symmetry  which  it  thus 
assumes,  is  in  harmony  with  the  average  equality  of  the 
actions  on  all  its  sides.  In  Di/flugia^  Fig.  134,  and  still 
better  in  Arcclla,  we  have  an  indefinitely-radial  symmetry 
occurring  where  the  conditions  are  different  above  and  below 
but  alike  all  around.  Among  the  Gregarinida  the  spherical 
symmetry  and  symmetry  passing  from  that  into  the  radial, 
are  such  as  appear  to  be  congruous  with  the  simple  cir- 
cumstances of  these  creatures  in  the  intestines  of  insects. 


170  MORPHOLOGICAL   DEVELOPMENT. 

But  the  relations  of  these  lowest  types  to  their  environments 
are  comparatively  so  indeterminate,  and  our  knowledge  of 


f 


their  actions  so  scanty,  that  little  heyond  negative  evidence 
can  be  expected  from  the  study  of  them. 

The  like  may  be  said  of  the  Infusoria.  These  are  more 
or  less  irregular.  In  some  cases  where  the  line  of  move- 
ment through  the  water  is  tolerably  definite  and  constant 
we  have  a  form  that  is  approximately  radial — externally  at 
Least.  But  usually,  as  shown  in  Figs.  137,  138,  139,  there  is 
either  an  unsymmetrical  or  an  asymmetrical  shape.  And  when 
one  of  these  creatures  is  watched  under  the  microscope,  the 
congruity  of  this  shape  with  the  incidence  of  forces  is  mani- 
fest. For  the  movements  are  conspicuously  varied  arid 
indeterminate — movements  which  do  not  expose  any  two 
or  more  sides  of  the  mass  to  approximately  equal  sets  of 
actions. 

§  245.  Among  aggregates  of  the  second  order,  as  among 
aggregates  of  the  first  order, we  fia  1  that  of  those  possessing 
any  definite  shapes  the  lowest  are  spherical  or  spheroidal. 
Such  are  the  Thalassicollce.  These  gelatinous  bodies  which 
float  passively  in  the  sea,  and  present  in  turn  all  their  sides 
to  the  same  influences,  have  their  parts  disposed  with  ap- 
proximate regularity  all  around  a  centre.  In  some  orders 
of  Foraminifera,  as  for  instance  the  Nummulites,  we  have 
secondary  aggregates  the  parts  of  which  are  spirally  ar- 
ranged, approximately  in  harmony  with  the  radial  relations 
of  the  society  to  the  environment ;  but  we  have  other  types 
in  which  the  congregated  units  are  distributed  in  ways  not 
easily  definable,  and  having  to  the  environment  relations  that 
are  obscure.  Further,  among  these  secondary  aggregates  in 
which  the  units,  only  physically  integrated,  have  not  had  their 


THE  GENERAL  SHAPES  OF  ANIMALS.          171 

individualities  merged  into  an  individuality  of  a  higher 
order,  must  be  named  the  compound  Infusoria.  The 
cluster  of  VorticellcB  in  Fig.  144,  will  sufficiently  exemplify 
them ;  and  the  striking  resemblance  borne  by  its  individuals 
to  those  of  a  radially-arranged  cluster  of  flowers,  will  show 
how,  under  analogous  conditions,  the  general  principles  of 
morphological  differentiation  are  similarly  illustrated  in  the 
two  kingdoms. 

§  246.  Radial  symmetry  is  usual  in  those  aggregates  of 
the  second  order  that  have  their  parts  sufficiently  differentiat- 
ed and  integrated  to  give  individualities  to  them  as  wholes. 
The  Ccelenterata  offer  numerous  examples  of  this.  Solitary 
polypes — hydroid  or  helianthoid — mostly  stationary,  and 
when  they  move,  moving  with  any  side  foremost,  do  not  by 
locomotion  subject  their  bodies  to  habitual  contrasts  of  con- 
ditions. Seated  with  their  mouths  upwards  or  downwards, 
or  else  at  all  degrees  of  inclination,  the  individuals  of  a 
species  taken  together,  are  subject  to  no  mechanical  actions 
affecting  some  parts  of  their  discs  more  than  other  parts. 
And  this  indeterminateness  of  attitude  similarly  prevents 
their  relations  to  prey  from  being  such  as  subject  some  of 
their  prehensile  organs  to  forces  unlike  those  to  which  the 
rest  are  subject.  The  fixed  end  is  differently  conditioned 
from  the  free  end,  and  the  two  are  therefore  different ;  but 
around  the  axis  running  from  the  fixed  to  the  free  end  the 
conditions  are  alike  in  all  directions,  and  the  form  therefore 
is  radial.  Again,  among  many  of  the  simple  free- 

swimming  Hydrozoa,  the  same  general  truth  is  exemplified 
under  other  circumstances.  In  a  common  Medusa,  advanc- 
ing through  the  water  by  the  rhythmical  contractions  of 
its  disc,  the  mechanical  reactions  are  the  same  on  all  sides  ; 
and  as,  from  accidental  causes,  every  part  of  the  edge  of  the 
disc  comes  upwards  in  its  turn,  no  part  is  permanently  af- 
fected in  a  different  way  from  the  rest.  Hence  the  radial 
form  continues. 


172  MORPHOLOGICAL   DEVELOPMENT 

In  others  of  this  same  group,  however,  there  occur  forms 
which  show  us  an  incipient  bilateralness  ;  and  help  us  to  see 
how  a  more  decided  bilateralness  may  arise.  Sundry  of  the 
Medusidce  are  proliferous,  giving  origin  to  gemmae  from  the 
body  of  the  central  polypite  or  from  certain  points  on  the 
edge  of  the  disc  ;  and  this  budding,  unless  it  occurs  equally 
on  all  sides,  which  it  does  not  and  is  unlikely  to  do,  must  tend 
to  destroy  the  balance  of  the  disc,  and  to  make  its  attitude 
less  changeable.  In  other  cases  the  growth  of  a  large  process 
from  the  edge  of  the  disc  on  one  side,  as  in  Steenstrupia,  Fig 
257 — a  process  that  is  perhaps  the  morphological  equivalent 
of  one  of  the  gemmae  just  named — constitutes  a  similar  modi- 
fication, and  a  cause  of  further  modification.  The  existence 
of  this  process  makes  the  animal  no  longer  divisible  into  any 
two  quite  similar  halves,  except  those  formed  by  a  plane 
passing  through  the  process ;  and  unless  the  process  is 
exactly  of  the  same  'specific  gravity  as  the  disc,  it  must  tend 
towards  either  the  lowest  or  the  highest  point,  and  must 
so  serve  to  increase  .the  bilateralness,  by  keeping  the  two 
sides  of  the  disc  similarly  conditioned  while  the  top  and 
bottom  are  differently  conditioned.  Fig.  258  represents  the 
underside  of  another  Medusa, 
in  which  a  more  decided  bi- 
lateralness is  produced  by  the 
presence  of  two  such  process- 
es. Among  the  simple 
free-swimming  Actinozoa,  occur 
like  deviations  from  radial  sym- 
metry, along  with  like  motions 
through  the  water  in  bilateral 
attitudes.  Of  this  a  Cydippe  is  a  familiar  example.  Though 
radial  in  some  of  its  characters,  as  in  the  distribution  of  its 
meridional  bands  of  locomotive  paddles  with  their  accompany- 
ing canals,  this  creature  has  a  two-sided  distribution  of 
tentacles  and  various  other  parts,  corresponding  with  its  two- 
sided  attitude  in  moving  through  the  water.  And  in  other 


THE    GENERAL    SHAPES    OF    ANIMALS.  173 

genera  of  this  group,  as  in  Cesium,  Eurhamphcea,  and 
Callianira,  that  almost  equal  distribution  of  parts  which 
characterizes  the  Beroe  is  quite  lost. 

Here  seems  a  fit  place  to  meet  the  objection  which  some 
may  feel  to  this  and  other  such  illustrations,  that  they  amount 
very  much  to  physical  truisms.  If  the  parts  of  a  Medusa 
are  disposed  in  radial  symmetry  around  the  axis  of  motion 
through  the  water,  there  will  of  course  be  no  means  of 
maintaining  one  part  of  its  edge  upwards  more  than  another ; 
and  the  equality  of  conditions  may  be  ascribed  to  the  radiate- 
ness,  as  much  as  the  radiateness  to  the  equality  of  conditions. 
Conversely,  when  the  parts  are  not  radially  arranged  round 
the  axis  of  motion,  they  must  gravitate  towards  some  one 
attitude,  implying  a  balance  on  the  two  sides  of  a  vertical 
plane — a  bilateralness ;  and  the  two-sided  conditions  so 
necessitated,  may  be  as  much  ascribed  to  the  bilateralness  as 
the  bilateralness  to  the  two-sided  conditions.  Doubt- 

less the  form  and  the  conditions  are,  in  the  way  alleged, 
necessary  correlates  ;  and  in  so  far  as  it  asserts  this,  the  ob- 
jection harmonizes  with  the  argument.  To  the  difficulty 
which  it  at  the  same  time  raises  by  the  implied  question — 
Why  make  the  form  the  result  of  the  conditions,  rather  than 
the  conditions  the  result  of  the  form  ?  the  reply  is  this  : — 
The  radial  type,  both  as  being  the  least  differentiated  type 
and  as  being  the  most  obviously  related  to  lower  typss,  must 
be  taken  as  antecedent  to  the  bilateral  type.  The  indi- 
vidual variations  which  incidental  circumstances  produce  in 
the  radial  type,  will  not  cause  divergence  of  a  species  from 
the  radial  type,  unless  such  variations  give  advantages  to  the 
individuals  displaying  them;  which  there  is  no  reason  to  sup- 
pose they  will  always  do.  Those  occasional  deviations  from 
the  radial  type,  which  the  law  of  the  instability  of  the  homo- 
geneous warrants  us  in  expecting  to  take  place,  will,  however, 
in  some  cases  be  beneficial ;  and  will  then  be  likely  to  estab- 
lish themselves.  "Such  deviations  must  tend  to  destroy  the 
original  indefiniteness  and  variability  of  attitude — must 


i?4  MORPHOLOGICAL    DEVELOPMENT. 

cause  gravitation  towards  an  habitual  attitude.  And  gravita- 
tion towards  an  habitual  attitude  having  once  commenced, 
will  continually  increase,  where  increase  of  it  is  not  negatived 
by  adverse  agencies :  each  further  degree  of  bilateralness 
rendering  more  decided  the  actions  that  conduce  to  bilateral- 
ness.  If  this  reply  be  thought  insufficient,  it  may  be  enforced 
by  the  further  one,  that  as,  among  plants,  the  incident  forces 
are  the  antecedents  and  the  forms  the  consequents  (changes  of 
forces  being  in  many  cases  visibly  followed  by  changes  of 
forms)  we  are  warranted  in  concluding  that  the  like  order  of 
cause  and  effect  holds  among  animals. 

§  247.  Keeping  to  the  same  type  but  passing  to  a  higher 
degree  of  composition,  we  meet  more  complex  and  varied 
illustrations  of  the  same  general  laws.  In  the  compound 
Ccelenterata,  presenting  clusters  of  individuals  that  are 
severally  homologous  with  the  solitary  individuals  last  dealt 
with,  we  have  to  note  both  the  shapes  of  the  individuals  thus 
united,  and  the  shapes  of  the  aggregates  made  up  of  them. 

Such  of  the  fixed  Hydrozoa  and  Actinozoa  as  form  branched 
societies,  continue  radial ;  both  because  their  varied  attitudes 
do  not  expose  them  to  appreciable  differences  in  their  rela- 
tions to  those  surrounding  actions  which  chiefly  concern 
them  (the  actions  of  prey),  and  because  such  differences,  even 
if  they  were  appreciable,  would  be  so  averaged  in  their 
effects  on  the  dissimilarly-placed  members  of  each  group  as 
to  be  neutralized  in  the  race.  Among  the 
tree-like  coral-polypedoms,  as  well  as  in 
such  ramified  assemblages  of  simpler  poly- 
pes as  are  shown  in  Figs.  149,  150,  we 
have,  indeed,  cases  in  many  respects  paral- 
lel to  the  cases  of  scattered  flowers  (§  233), 
which  though  placed  laterally  remain  radial, 
because  no  differentiating  agency  can  act 
uniformly  on  all  of  them.  Meanwhile,  in  the  groups 

which  these  united  individuals  compose,  we  see  the  shapes  of 


THE  GENERAL  SHAPES  OF  ANIMALS.          175 

plants  further  simulated  under  a  further  parallelism  of  con- 
ditions. The  attached  ends  differ  from  the  free  ends  as  they 
do  in  plants  ;  and  the  regular  or  irregular  branches  obvious- 
ly stand  to  environing  actions  in  relations  analogous  to  those 
in  which  the  branches  of  plants  stand. 

The  members  of  those  compound  Ccelenterata  which  move 
through  the  water  by  their  own  actions,  in  attitudes  that  are 
approximately  constant,  show  us  a  more  or  less  distinct  two- 
sidedness.  Diphyes,  Fig.  259,  furnishes  an  example.  Each 


of  the  largely-developed  and  modified  polypites  forming  its 
swimming  sacs  is  bilateral,  in  correspondence  with  the  bi- 
lateralness  of  its  conditions ;  and  in  each  of  the  appended 
polypites  the  insertion  of  the  solitary  tentacle  produces  a 
kindred  divergence  from  the  primitive  radial  type.  The 

aggregate,  too,  which  here  very  much  subordinates  its  mem- 
bers, exhibits  the  same  conformity  of  structure  to  circum- 
stances. It  admits  of  symmetrical  bisection  by  a  plane  pass- 
ing through  its  two  contractile  sacs,  or  nectocalyces,  but  not 
by  any  other  plane  ;  and  the  plane  which  thus  symmetrically 
bisects  it-,  is  the  vertical  plane  on  the  two  sides  of  which  its 
parts  are  similarly  conditioned  as  it  propels  itself  through 
the  water. 

Another  group  of  the  oceanic  Uydrozoa,  the  PhysophoridcB, 
furnishes  interesting  evidence — not  so  much  in  respect  of  the 
forms  of  the  united  individuals,  which  we  may  pass  over,  as 
in  respect  of  the  forms  of  the  aggregates.  Some  of  these 
which  are  without  swimming  organs,  have  their  parts  sus- 
pended from  air-vessels  which  habitually  float  on  the  surface 
of  the  water ;  and  the  distribution  of  their  parts  is  asyra- 


176 


MORPHOLOGICAL    DEVELOPMENT. 


metrical.  The  Physalia,  Fig.  152,  is  an  example.  Here  the 
relations  of  the  integrated  group  of  individuals  to  the  environ- 
ment are  indefinite ;  and  there  is  hence  no  agency  tending 
to  change  that  comparatively  irregular  mode  of  growth  that 
is  probably  derived  from  a  primordial 
type  of  the  branched  Hydrozoa. 

So  various  are  the  modes  of  union 
'among  the  compound  Ccelenterata,  that 
it  is  out  of  the  question  to  deal  with 
them  all.  Even  did  space  permit,  it 
would  be  impracticable  for  any  one  but 
a  professed  naturalist,  to  trace  through- 
out this  group  the  relations  between 
shapes  and  conditions  of  existence.  The 
above  must  be  taken  simply  as  a  few  of 
the  most  significant  and  easily- interpret- 
able  cases. 

'•**  §  248.  In  the   sub-kingdom   Motlus- 

coida,  we  meet  with  examples  not  wholly 
unlike  the  foregoing.  Among  the  types  assembled  under 
this  title  there  are  simple  individuals  or  aggregates  of  the 
second  order,  and  societies  or  tertiary  aggregates  produced 
by  their  union.  The  relations  of  forms  to  forces  have  to  be 
traced  in  both. 

Solitary  Ascidians,  fixed  or  floating,  carry  on  an  inactive 
and  indefinite  converse  with  the  actions  in  the  environment. 
Without  power  to  move  about  vivaciously,  and  unable  to 
catch  any  prey  but  that  contained  in  the  currents  of  water 
they  absorb  and  expel,  these  creatures  are  not  exposed  to 
sets  of  forces  that  are  equal  on  two  or  more  sides  ;  and  their 
shapes  consequently  remain  vague.  Though  there  are  in 
them  traces  of  symmetrical  arrangement,  probably  due  to 
their  derivation,  yet  they  are  substantially  asymmetrical. 
Fig.  156  is  an  example.  Among  the  composite 

Ascidians,  floating   and  fixed,  the  shape  of  the  aggregate, 


THE    GENERAL    SHAPES    OF    ANIMALS.  17? 

partly  determined  by  the  habitual  mode  of  gemmation  and 
partly  by  the  surrounding  conditions  in  each  case,  is  in 
great  measure  indefinite.  We  can  say  no  more  about  it  than 
that  it  is  not  obviously  at  variance  with  the  laws  alleged. 

Evidence  of  a  more  positive  kind  occurs  among  those  com- 
pound Molluscoida  which  are  most  like  the  compound 
Coehnterata  in  their  modes  of  union — the  Polyzoa.  Many  of 
these  form  groups  that  are  more  or  less  irregular — spreading 
as  films  over  solid  surfaces,  combining  into  sea-weed-like 
fronds,  budding  out  from  creeping  stolons,  or  growing  up 
into  tree- shaped  societies ;  and  besides  aggregating  ir- 
regularly they  are  irregularly  placed  on  surfaces  inclined  in 
all  directions.  Merely  noting  that  this  asymmetrical  dis- 
tribution of  the  united  individuals  is  explained  by  the 
absence  of  definiteness  in  the  relations  of  the  aggregate  to 
incident  forces,  it  concerns  us  chiefly  to  observe  that  the 
united  individuals  severally  exemplify  the  same  truth  as  do 
similarly- united  individuals  among  the  Ccclenterata.  While 
their  internal  organs,  though  said  to  have  a  trace  of  bi- 
luteralness,  cannot  be  said  to  display  any  definite  symmetry  ; 
their  external  organs  are  completely  radial.  Averaging  the 
members  of  each  society,  the  ciliated  tentacles  they  protrude 
are  similarly  related  to  prey  on  all  sides ;  and  therefore 
remain  the  same  on  all  sides.  This  distribution  of  tentacles 
is  not,  however,  without  exception.  Among  the  fresh-water 
Polyzoa  there  are  some  genera,  as  Plumatella  and  Crystatella, 
in  which  the  arrangement  of  these  parts  is  very  decidedly 
bilateral.  Some  species  of  them  show  us  such  relations  of 
the  individuals  to  one  another  and  to  their  surface  of  attach- 
ment, as  give  a  clue  to  this  modification  ;  but  in  other  species 
the  meaning  of  this  deviation  from  the  radial  type  is  not 
obvious. 

§  249.  In  that  somewhat  heterogeneous  assemblage  of 
animals  now  classed,  perhaps  provisionally,  as  Annuloida,  we 
begin  again  with  simple  aggregates  of  the  second  order,  and 


178  MORPHOLOGICAL    DEVELOPMENT. 

ascend  to  aggregates  in  which,  we  have  seen  reason  to  suspect 
a  higher  degree  of  composition.  Good  examples  of  the  con- 
nexions between  forms  and  forces  occur  in  this  group. 

Among  the  lower  annuloid  types,  the  Planaria  exemplifies 
the  single  bilateral  symmetry  which,  even  in  very  inferior 
forms,  accompanies  the  habit  of  moving  in  one  direction  over 
a  solid  surface.  Humbly  organized  as  are  these  creatures 
and  their  allies  the  Nemertidce,  we  see  in  them  just  as  clearly 
as  in  the  highest  animals,  that  where  the  movements  subject 
the  body  to  different  forces  at  its  two  ends,  different  forces 
on  its  under  and  upper  surfaces,  and  like  forces  along  its  two 
sides,  there  arises  a  corresponding  form,  unlike  at  its  extremi- 
ties, unlike  above  and  below,  but  having  its  two  sides  alike. 

The  Echinodermata  furnish  us  with  instructive  illustrations 
— instructive  because  among  types  that  are  nearly  allied,  we 
meet  with  wide  deviations  of  form  answering  to  marked  con- 
trasts in  the  relations  to  the  environment.  The  facts  fall 
into  four  groups.  The  Crinoidea,  once  so  abundant 

and  now  so  rare,  present  a  radial  symmetry  answering  to 
an  incidence  of  forces  that  is  equal  on  every  side.  In  the 
general  attitudes  of  their  parts  towards  surrounding  actions, 
ihey  are  like  uniaxial  plants  or  like  polypes ;  and  show,  as 
they  do,  marked  differences  between  the  attached  ends  and 
the  free  ends,  along  with  even  distributions  of  parts  all  round 
their  axes.  In  the  Ophiuridea,  proved  to  be  near 

akin  to  the  Crinoids,  and  in  the  Star-fishes,  we  have  radial 
symmetry  co-existing  with  very  different  habits  ;  but  habits 
which  nevertheless  account  for  the  maintenance  of  the  form. 
Holding  on  to  rocks  and  weeds  by  its  simple  or  branched 
arms,  or  by  the  suckers  borne  on  the  under  surface  of  its 
rays,  one  of  these  creatures  moves  about  not  always  with  one 
side  foremost,  but  with  any  side  foremost.  Consequently, 
averaging  its  movements,  its  amis  or  rays  are  equally  af- 
fected, and  therefore  remain  the  same  on  all  sides.  On 
watching  the  ways  of  the  common  Sea-urchin,  we  aro 
similarly  furnished  with  an  explanation  of  its  spherical,  or 


THE   GENERAL   SHAPES   OF   AXIMALS.  179 

rather  its  spheroidal,  figure.  Here  the  habit  is  not  to  move 
over  any  one  approximately-flat  surface  ;  but  the  habit  is  to 
hold  on  by  several  surfaces  on  different  sides  at  the  same 
time,  Frequenting  crevices  and  the  interstices  among  stones 
and  weeds,  the  Sea-urchin  protrudes  the  suckers  arranged 
in  meridional  bands  over  its  shell,  laying  hold  of  objects  now 
on  this  side  and  now  on  that,  now  above  and  now  below : 
the  result  being,  that  it  does  not  move  in  all  directions  over 
one  plane  but  in  all  directions  through  space.  Hence  the 
approach  in  general  form  towards  spherical  symmetry — an 
approach  which  is,  however,  restrained  by  the  relations  e* 
the  parts  to  the  mouth  and  vent :  the  conditions  not  being 
exactly  the  same  at  the  two  poles  as  at  other  parts  of  the 
surface.  Still  more  significant  is  that  deviation 

from  this  shape  which  occurs  among  such  of  the  Echinidea 
as  have  habitats  of  a  different  kind,  and,  consequently,  dif- 
ferent habits.  The  genera  Echinocyamus,  Spatangus,  Bris- 
sus,  and  Amphidotus,  diverge  markedly  towards  a  bilateral 
structure.  These  creatures  are  found  not  on  rocky  shores 
but  on  flat  sea-bottoms,  and  some  of  them  only  on  bottom? 
of  sand  or  mud.  Here,  there  is  none  of  that  distribution  of 
surfaces  on  all  sides  which  makes  the  spheroidal  form  con- 
gruous with  the  conditions.  Having  to  move  about  over  an 
approximately-horizontal  plane,  any  deviation  of  structure 
which  leads  to  one  side  being  kept  always  foremost,  will  be 
an  advantage :  greater  fitness  to  function  becoming  possible 
in  proportion  as  function  becomes  fixed.  Survival  of  the 
fittest  will  therefore  tend  to  establish,  under  such  conditions, 
a  form  that  keeps  the  same  part  in  advance — a  form  in 
which,  consequently,  the  original  radial  symmetry  diverges 
more  and  more  towards  bilateral  symmetry.  It  may 

be  well  to  add  that  the  validity  of  these  interpretations  does 
not  depend  on  the  view  taken  of  the  alliances  of  the  Echino- 
derms,  atid  their  primitive  type  of  symmetry.     If,  as  Pro 
fessor  Huxley  contends,  the  Echinoderms,  having  bilateral 
larvae,  cannot  be  held  akin  to  those  lower  typos  in  which  the 


180  MORPHOLOGICAL   DEVELOPMENT. 

radial  structure  is  constant  and  complete ;  it  do33  not  follow 
that  the  above  reasonings  are  erroneous.  On.  the  contrary 
the  derivation  of  these  radially-symmetrical  forms  from  forms 
not  radially-symmetrical,  would  show  how  entirely  the 
structure  of  the  organism  is  moulded  by  the  distribution  of 
forces  to  which  its  mode  of  life  exposes  it. 

The  remaining  Aimutoida,  most  of  them  parasitic,  must 
be  passed  over.  Living  within  the  bodies  of  'other  creatures, 
they  have  their  forms  determined  by  conditions  that  are  too 
obscure  to  be  satisfactorily  dealt  with. 

§  250.  Very  definite  and  comparatively  uniform,  are  the 
relations  between  shapes  and  circumstances  among  the 
Annulosa — including  under  that  title  the  Annelida  and  the 
Articulata.  The  agreements  and  the  disagreements  are 
equally  instructive. 

At  one  time  or  other  of  its  life,  if  not  throughout  its  life, 
every  annulose  animal  is  locomotive ;  and  its  temporary 
or  permanent  locomotion,  being  carried  on  with  one  end 
habitually  foremost  and  one  surface  habitually  uppermost, 
it  fulfils  those  conditions  under  which  bilateral  symmetry 
arises.  Accordingly,  bilateral  symmetry  is  traceable  through- 
out the  whole  of  this  sub-kingdom.  Traceable,  we  must 
say,  because,  though  it  is  extremely  conspicuous  in  the 
immense  majority  of  annulose  types,  it  is  to  a  consider- 
able extent  obscured  where  obscuration  is  to  be  expected. 
The  embryos  of  the  Tubicoke,  after  swimming  about 
awhile,  settle  down  and  build  themselves  tubes,  from  which 
they  protrude  their  heads ;  and  in  them,  or  in  some  of 
them,  the  bilateral  symmetry  is  disguised  by  the  develop- 
ment of  head-appendages  in  an  all-sided  manner.  The 
tentacles  of  Terebella  are  distributed  much  in  the  same  way 
as  those  of  a  polype.  The  breathing  organs  in  Sabella 
un'spira,  Fig.  260,  do  not  correspond  on  opposite  sides  of  a 
median  plane.  Even  here,  however,  the  body  retains  its 
primitive  bilateralness ;  and  it  is  further  to  be  remarked  that 


THE    GENERAL    SHAPES    OF    ANIMALS. 


181 


this  loss  of  bilateralness  in  the  external  appendages,  does  not 
occur  where  the  relations  to  external  conditions  continue 
bilateral :  witness  the  Serpula,  Fig.  261,  which  has  its 


respiratory  tufts  arranged  in  a  two-sided  way,  under  the 
two-sided  conditions  involved  by  the  habitual  position  of 
its  tube. 

The  community  of  symmetry  among  the  higher  A.nnulosa, 
has  an  unobserved  significance.  That  Flies,  Beetles,  Lob- 
sters, Centipedes,  Spiders,  Mites,  have  in  common  the 
characters,  that  the  end  which  moves  in  advance  differs  from 
the  hinder  end,  that  the  upper  surface  differs  from  the  under 
surface,  and  that  the  two  sides  are  alike,  is  a  truth  received  as 
a  matter  of  course.  After  all  that  has  been  said  above,  how- 
ever, it  will  be  seen  to  have  a  meaning  not  to  be  overlooked  ; 
since  it  supplies  a  million-fold  illustration  of  the  laws  that 
have  been  se^  forth.  It  is  needless  to  give  diagrams.  Every 
reader  can  call  to  mind  the  unity  indicated. 

"While,  however,  annulose  animals  repeat  so  uniformly 
these  traits  of  structure,  there  are  certain  other  traits  in 
which  they  are  variously  contrasted;  and  their  contrasts 
have  to  be  here  noted,  as  serving  further  to  build  up  the 
general  argument.  In  them  we  see  the  stages  through 
which  bilateral  symmetry  becomes  gradually  more  marked,  as 
the  conditions  it  responds  to  become  more  decided.  A 


182 


MORPHOLOGICAL   DEVELOPMENT. 


common  Earth-worm  may  be  instanced  as  a  member  of 
this  sub-kingdom  that  is  among  the  least-conspicuously 
bilateral.  Though  internally  its  parts  have  a  two-sided 
arrangement  ;  and  though  the  positions  of  its  orifices  give  it 
an  external  two-sidedness,  at  the  same  time  that  they  estab- 
lish a  difference  between  the  two  ends  ;  yet  its  two-sidedness 
is  not  strongly  marked.  The  form  deviates  but  little  from 
what  we  have  distinguished  as  triple  bilateral  symmetry  :  if 
the  creature  is  cut  across  the  middle,  the  head  and  tail  ends 
are  very  much  alike  ;  if  cut  in  two  along  its  axis  by  a  hori- 
zontal plane,  the  under  and  upper  halves  are  very  much 
alike  ;  and  if  cut  in  two  along  its  axis  by  a  vertical  plane, 
the  two  sides  are  quite  alike.  Figs.  263  and  264  will  make 
this  clear.  Such  creatures  as  the  Julus  and  the 

Centipede,  may  be  taken  as  showing  a  transition  to  double 
bilateral  symmetry.  Besides  being  divisible  into  exactly 
similar  halves  by  a  vertical  plane  passing  through  the  axis, 
one  of  these  animals  may  be  bisected  transversely  into  parts 
that  differ  only  slightly  ;  but  if  cut  in  two  by  a  horizontal 
plane  passing  through  the  axis,  the  under  and  upper  halves 
are  decidedly  unlike.  Figs.  265,  266,  exhibit  these 
traits,  Among  the  isopodous  crustaceans,  the  departure 

from   these  low  types  of  symmetry  is   more   marked.     As 


Z**  I  ,     Zf-4 

-gsraxxnxxxxxx^^  -  -4-  -^jv 


shown  in  Figs.  267  and  268,  the  contrast  between  the  upper 
and  under  parts  is  greater,  and  the  head  and  tail  ends  differ 


THE    GENERAL    SHAPES    OF    ANIMALS.  183 

more  obviously.  In  all  the  higher  Articulata,  the 

unlikeness  between  the  front  half  and  the  hind  half  has 
become  conspicuous :  there  is  in  them  single  bilateral 
symmetry  of  so  pronounced  a  kind,  that  no  other  resem- 
blance is  suggested  than  that  between  the  two  sides.  By 
Figs.  269  and  270,  representing  a  decapodous  crustacean 
divided  longitudinally  and  transversely,  this  truth  is  made 
manifest.  On  calling  to  mind  the  habits  of  the 

creatures  here  drawn  and  described,  it  will  be  seen  that 
they  explain  these  forms.  The  incidence  of  forces  is  the 
same  all  around  the  Earth-worm  as  it  burrows  through  the 
compact  ground.  The  Centipede,  creeping  amid  loose  soil  or 
debris  or  beneath  stones,  insinuates  itself  between  solid  sur- 
faces— the  interstices  being  mostly  greater  in  one  dimension 
than  in  others.  And  all  the  higher  Anmdosa,  moving  about 
as  they  do  over  exposed  objects,  have  their  dorsal  and 
ventral  parts  as  dissimilarly  acted  upon  as  are  their  two  ends. 
One  other  fact  only  respecting  annulose  animals  needs  to 
be  noticed  under  this  head — the  fact,  namely,  that  they 
become  unsymmetrical  where  their  parts  are  unsymmetrically 
related  to  the  environment.  The  common  Hermit-crab 
serves  as  an  instance.  Here,  in  addition  to  the  unlikeness  of 
the  two  sides  implied  by  that  curvature  of  the  body  which  fits 
the  creature  to  the  shell  it  inhabits,  there  is  an  unlikeness 
due  to  the  greater  development  of  the  limbs,  and  especially 
the  claws,  on  the  outer  side.  As  in  the  embryo  of  the 
Hermit-crab  the  two  sides  are  alike ;  and  as  the  embryo  may 
be  taken  to  represent  the  type  from  which  the 
Hermit-crab  has  been  derived ;  we  have  in 
this  case  evidence  that  a  symmetrically-bi- 
lateral form  has  been  moulded  into  an  unsym- 
metrically-bilateral  form,  by  the  action  of  un- 
symmetrically-bilateral  conditions.  A  further 
illustration  is  supplied  by  Bopyrus,  Fig.  271  : 
a  parasite  the  habits  of  which  similarly  account  for  its  dis- 
torted shape. 


184  MORPHOLOGICAL    DEVELOPMENT. 

§  251.  Among  the  Mollusca  we  find  more  varied  relations 
between  shapes  and  circumstances.  Some  of  them  are 
highly  instructive. 

Hollusks  of  one  order,  the  Ptcropoda,  swim  in  the  sea 
much  in  the  same  way  that  butterflies  fly  in 
the  air,  and  have  shapes  not  altogether  unlike 
those  of  butterflies.  Fig.  272  represents  one 
of  these  creatures.  That  its  bilaterally- sym- 
metrical shape  harmonizes  with  itsbilaterally- 
symmetrical  conditions  is  sufficiently  obvious. 
Among  the  Lamcllibranchiata,  we  have 
diverse  forms  accompanying  diverse  modes  of 
life.  Such  of  them  as  frequently  move  about,  like  the  fresh- 
water Mussel,  have  their  two  valves  and  the  contained  parts 
alike  on  the  opposite  sides  of  a  vertical  plane :  they  are 
bilaterally  symmetrical  in  conformity  with  their  mode  of 
movement.  The  marine  M\issel,  too,  though  habitually 
fixed,  and  though  not  usually  so  fixed  that  its  two  valves  are 
similarly  conditioned,  still  retains  that  bilateral  symmetry 
which  is  characteristic  of  the  order ;  and  it  does  this  because 
in  the  species  considered  as  a  whole,  the  two  valves  are  not 
dissimilarly  conditioned.  If  the  positions  of  the  various 
individuals  are  averaged,  it  will  be  seen  that  the  differenti- 
ating actions  neutralize  one  another.  In  certain 
other  fixed  Lamellibranchs,  however,  there  is  a  considerable 
deviation  from  bilateral  symmetry  ;  and  it  is  a  deviation  of 
the  kind  to  be  anticipated  under  the  circumstances.  Where 
one  valve  is  always  downwards,  or  next  to  the  surface  of 
attachment,  while  the  other  valve  is  always  upwards,  or  next 
to  the  environing  water,  we  may  expect  to  find  the  two 
valves  become  unlike.  This  we  do  find  :  witness  the  Oyster. 
In  the  Oyster,  too,  we  see  a  further  irregularity.  There  is  a 
great  indefiniteness  of  outline,  both  in  the  shell  and  in  the 
animal  —  an  indefiniteness  made  manifest  by  comparing 
different  individuals.  We  have  but  to  remember  that  growing 
clustered  together,  as  Oysters  do,  they  must  interfere  with 


THE   GENERAL   SHAPES    OF   ANIMALS.  185 

one  another  In  various  -ways  and  degrees,  to  see  how  the 
mdeterminateness  of  form  and  the  variety  of  form  are 
accounted  for. 

Among  the  Gasteropods,  modifications  of  a  more  definite 
kind  occur.  "In  all  Mollusks,"  says  Professor  Huxley, 
"  the  axis  of  the  hody  is  at  first  straight,  and  its  parts  are 
arranged  symmetrically  with  regard  to  a  longitudinal  verti- 
cal plane,  just  as  in  a  vertebrate  or  an  articulate  embryo." 
In  some  Gasteropods,  as  the  Chiton,  this  bilateral  sym- 
metry is  retained — the  relations  of  the  body  to  surround- 
ing actions  not  being  such  as  to  disturb  it.  But  in  those 
more  numerous  types  that  have  spiral  shells,  there  is  a 
marked  deviation  from  bilateral  symmetry,  as  might  be  ex- 
pected. "  This  asymmetrical  over-development  never  affects 
the  head  or  foot  of  the  mollusk  :  "  only  those  parts  which, 
by  inclosure  in  a  shell,  are  protected  from  environing  actions, 
lose  their  bilateralness ;  while  the  external  parts,  subjected  by 
the  movements  of  the  creatures  to  bilateral  conditions,  remain 
bilateral.  Here,  however,  a  difficulty  meets  us.  Why  is  it 
that  the  naked  Gasteropods,  such  as  our  common  slugs, 
deviate  from  bilateral  symmetry,  though  their  modes  of 
movement  are  those  along  with  which  complete  bilateral 
symmetry  usually  occurs  ?  The  reply  is,  that  their  devia- 
tions from  bilateral  symmetry  are  probably  inherited,  and 
that  they  are  maintained  in  such  parts  of  their  organiza- 
tion as  are  not  exposed  to  bilaterally- symmetrical  conditions. 
There  is  reason  to  believe  that  the  naked  Gasteropods  are 
descended  from  Gasteropods  that  had  shells :  the  evidence 
being  that  the  naked  Gasteropods  have  shells  during  the 
early  stages  of  their  development,  and  that  some  of  them 
retain  rudimentary  shells  throughout  life.  Now  the  shelled 
Gasteropods  deviate  from  bilateral  symmetry  in  the  dis- 
position of  both  the  alimentary  system  and  the  reproductive 
system.  The  naked  Gasteropods,  in  losing  their  shells,  have 
lost  that  immense  one-sided  development  of  the  alimentary 
system  which  fitted  them  to  their  shells,  and  have  acquirel 


186  MORPHOLOGICAL    DEVELOPMENT 

that  bilateral  symmetry  of  external  figure  which  fits  them 
for  their  habits  of  locomotion  ;  but  the  reproductive  system 
remains  one-sided,  because,  in  respect  to  it,  the  relations  to 
external  conditions  remain  one-sided. 

The  Cephalopods,  which  are  interpretable  as  higher  de- 
velopments on  the  Grasteropod  type,  show  us  bilaterally-sym- 
metrical external  forms  along  with  habits  of  movement  through 
the  water  in  two-sided  attitudes.  At  the  same  time,  in  the  radial 
distribution  of  the  arms,  enabling  one  of  these  creatures  to 
take  an  all- sided  grasp  of  its  prey,  we  see  how  readily  upon  one 
kind  of  symmetry  there  may  be  partially  developed  another 
kind  of  symmetry,  where  the  relations  to  conditions  favour  it. 

§  252.  The  Vertebrata  illustrate  afresh  the  truths  which 
we  have  already  traced  among  the  Annulosa.  Flying 
through  the  air,  swimming  through  the  water,  and  running 
over  the  earth  as  vertebrate  animals  do,  in  common  with 
annulose  animals,  they  are,  in  common  with  annulose  ani- 
mals, different  at  their  anterior  and  posterior  ends,  different 
at  their  dorsal  and  ventral  surfaces,  but  alike  along  their 
two  sides.  This  single  bilateral  symmetry  remains  constant 
under  the  extremest  modifications  of  form.  Among  fish 
we  see  it  alike  in  the  horizontally-flattened  Skate,  in  the 
vertically-flattened  Bream,  in  the  almost  spherical  Diodon, 
and  in  the  greatly- elongated  Syngnathus.  Among  reptiles 
the  Turtle,  the  Snake,  and  the  Crocodile  all  display  it.  And 
under  the  countless  modifications  of  structure  displayed  by 
birds  and  mammals,  it  remains  conspicuous. 

A  less  obvious  fact  which  it  concerns  us  to  note  among  the 
Vertebrata,  parallel  to  one  which  we  noted  among  the 
Annulosa,  is  that  whereas  the  lower  vertebrate  forms  deviate 
but  little  from  triple  bilateral  symmetry,  the  deviation  be- 
comes great  as  we  ascend.  Figs.  273  and  274  show  how, 
besides  being  divisible  into  similar  halves  by  a  vertical  plane 
passing  through  its  axis,  a  Fish  is  divisible  into  halves  that 
are  not  very  dissimilar  by  a  horizontal  plane  passing  through 


THE    GENERAL    SHAPES    OF    ANIMALS. 


is; 


its  axis,  and  also  into  other  not  very  dissimilar  halves  by  a 
plane  cutting  it  transversely.  If,  as  shown  in  Figs.  275 
and  276,  analogous  sections  be  made  of  a  superior  Reptile,  the 
divided  parts  differ  more  decidedly.  When  a  Mammal  and  a 
Bird  are  treated  in  the  same  way,  as  shown  in  Figs.  277, 
278,  and  Figs.  279,  280,  the  parts  marked  off  by  the  divid- 


ing  planes  are  unlike  in  far  greater  degrees.  On  considering 
the  mechanical  converse  between  organisms  of  these  several 
types  and  their  environments — on  remembering  that  the 
fish  habitually  moves  through  a  homogeneous  medium  of 
nearly  the  same  specific  gravity  as  itself,  that  the  terrestrial 
reptile  either  crawls  on  the  surface  or  raises  itself  very  in- 
completely above  it,  that  the  more  active  mammal,  having 


188  MORPHOLOGICAL    DEVELOPMENT. 

its  supporting  parts  more  fully  developed,  therebv  lias  the 
under  half  of  its  body  made  more  different  from  the  upper 
half,  and  that  the  bird  is  subject  by  its  mode  of  life  to  yet 
another  set  of  actions  and  reactions ;  we  shall  see  that  these 
facts  are  quite  congruous  with  the  general  doctrine,  and  fur- 
nish further  support  to  it. 

One  other  significant  piece  of  evidence  must  be  named. 
Among  the  Annulosa  we  found  unsymmetrical  bilateralness 
in  creatures  having  habits  exposing  them  to  unlike  conditions 
on  their  two  sides;  and  among  the  Vertebrata  we  find  parallel 
cases.  They  are  presented  by  the  Pleuronectidce — the  order 
of  distorted  flat  fishes  to  which  the  Sole  and  the  Flounder 
belong.  On  the  hypothesis  of  evolution,  we  must  conclude 
that  fishes  of  this  order  have  arisen  from  an  ordinary  bila- 
terally-symmetrical type  of  fish,  which,  feeding  at  the 
bottom  of  the  sea,  gained  some  advantage  by  placing  itself 
with  one  of  its  sides  downwards,  instead  of  maintaining  the 
vertical  attitude.  Besides  the  general  reason  there  are  speci- 
fic reasons  for  concluding  this.  In  the  first  place,  the  young 
Role  or  Flounder  is  bilaterally  symmetrical — has  its  eyes  on 
opposite  sides  of  its  head,  and  swims  in  the  usual  way.  In  the 
second  place,  the  metamorphosis  which  produces  the  unsym- 
metrical structure  sometimes  does  not  take  place — there  are 
abnormal  Flounders  that  swim  vertically,  like  other  fishes. 
In  the  third  place,  the  transition  from  the  symmetrical 
structure  to  the  unsymmetrical  structure  may  be  traced. 
Almost  incredible  though  it  seems,  one  of  the  eyes  is 
transferred  from  the  under-side  of  the  head  to  the  upper- 
side.  Until  lately  it  was  supposed  that  the  change  by 
which  the  two  eyes,  originally  placed  on  opposite  sides,  come 
to  be  placed  on  the  same  side,  is  effected  by  a  distortion 
of  the  cranium  ;  but  it  is  now  asserted  that  actual  migration 
of  an  eye  occurs.  According  to  Prof.  Steenstrup,  the  eye 
passes  between  the  ununited  bones  of  the  skull ;  but  according 
to  Prof.  Thomson,  it  passes  under  the  skin.  Be  the  course  of 
the  metamorphosis  what  it  may,  however,  it  furnishes  several 


THE    GENERAL    SHARES   OF    ANIMALS.  189 

remarkable  illustrations  of  the  way  in  which  forms  become 
moulded  into  harmony  with  incident  forces.  For  besides 
this  divergence  from  bilateral  symmetry  involved  by  the 
presence  of  both  eyes  upon  the  upper  side,  there  is  a  further 
livergence  from  bilateral  symmetry  involved  by  the  differ- 
entiation of  the  two  sides  in  respect  to  the  contours  of  their 
surfaces  and  the  sizes  of  their  fins.  And  then,  what  is  still 
moi-e  significant,  there  is  a  near  approach  to  likeness  be- 
tween the  halves  that  were  originally  unlike,  but  are,  under 
the  new  circumstances,  exposed  to  like  conditions.  The 
body  is  divisible  into  similarly- shaped  parts  by  a  plane 
cutting  it  along  the  side  from  head  to  tail:  "  the  dorsal  and 
ventral  instead  of  the  lateral  halves  become  symmetrical  in 
outline  and  are  equipoised." 

§  253.  Thus,  little  as  there  seems  in  common  between  the 
shapes  of  plants  and  the  shapes  of  animals,  we  yet  find,  on 
analysis,  that  the  same  general  truths  are  displayed  by 
both.  The  one  ultimate  principle  that  in  any  organism 
equal  amounts  of  growth  take  place  in  those  directions  in 
which  the  incident  forces  are  equal,  serves  as  a  key  to  the 
phenomena  of  morphological  differentiation.  By  it  we  are 
furnished  with  interpretations  of  those  likenesses  and  un- 
likenesses  of  parts,  which  are  exhibited  in  the  several  kinds 
of  symmetry ;  and  when  we  take  into  account  inherited 
effects,  wrought  under  ancestral  conditions  contrasted  in 
various  ways  with  present  conditions,  we  are  enabled  to 
comprehend,  in  a  general  way,  the  actions  by  which  animals 
have  been  moulded  into  the  shapes  they  possess. 

To  fill  up  the  outline  of  the  argument,  so  as  to  make  it 
correspond  throughout  with  the  argument  respecting  vegetal 
forms,  it  would  be  proper  here  to  devote  a  chapter  to  the 
differentiations  of  those  homologous  segments  out  of  which 
animals  of  certain  types  are  composed.  Though,  among  most 
animals  of  the  third  degree  of  composition,  such  as  the  root- 
ed Hydrozoa,  the  Polyzoa,  and  the  Ascidioida,  the  united 


190  MORPHOLOGICAL    DEVELOPMENT. 

individuals  are  not  reduced  to  the  condition  of  segments  of  a 
composite  individual,  and  do  not  display  any  marked  differ- 
entiations ;  yet  there  are  some  animals  in  which  such 
subordinations,  and  consequent  heterogeneities,  occur.  The 
oceanic  Hydrozoa  form  one  group ;  and  we  have  seen 
reason  to  conclude  that  the  Annulosa  form  another  group. 
It  is  not  worth  while,  however,  to  occupy  space  in  detailing 
these  unlikenesses  of  homologous  segments,  and  seeking 
specific  explanations  of  them.  Among  the  oceanic  Hydrozoa 
they  are  extremely  varied  ;  and  the  habits  and  derivations  of 
these  creatures  are  so  little  known,  that  there  are  no  adequate 
data  for  interpreting  the  forms  of  the  parts  in  terms  of  their 
relations  to  the  environment.  Conversely,  among  the  .4/1- 
nulosa  those  differentiations  of  the  homologous  segments 
which  accompany  their  progressing  integration,  have  so 
much  in  common,  and  have  general  causes  which  are  so  ob- 
vious, that  it  is  needless  to  deal  with  them  at  any  length. 
They  are  all  explicable  as  due  to  the  exposure  of  different  part 3 
of  the  chain  of  segments  to  different  sets  of  actions  and  re- 
actions :  the  most  general  contrast  being  that  between  the 
anterior  segments  and  the  posterior  segments,  answering  to 
the  most  general  contrast  of  conditions  to  which  annulose 
animals  subject  their  segments ;  and  the  more  special  con- 
trasts answering  to  the  contrasts  of  conditions  entailed  by 
their  more  special  habits. 

Were  an  exhaustive  treatment  of  the  subject  practicable, 
there  should  here,  also,  come  a  chapter  devoted  to  the  internal 
structures  of  animals — meaning,  more  especially,  the  shapes 
and  arrangements  of  the  viscera.  The  relations  between 
forms  and  forces  among  these  inclosed  parts,  are,  however, 
mostly  too  obscure  to  allow  of  interpretation.  Protected  as 
the  viscera  are  in  great  measure  from  the  incidence  of  ex- 
ternal forces,  we  are  not  likely  to  find  much  correspondency 
between  their  distribution  and  the  distribution  of  external 
forces.  In  this  case  the  influences,  partly  mechanical,  partly 
physiological,  which  the  organs  exercise  on  one  another, 


THE    GENERAL    SHAPES   OF    ANIMALS.  191 

become  the  chief  causes  of  their  changes  of  figure  and  ar- 
rangement ;  and  these  influences  are  complex  and  indefinite. 
One  general  fact  may,  indeed,  be  noted — the  fact,  namely, 
that  the  divergence  towards  asymmetry  which  generally 
characterizes  the  viscera,  is  marked  among  those  of  them 
which  are  most  removed  from  mechanical  converse  with  the 
environment,  but  not  so  marked  among  those  of  them  which 
are  less  removed  from  such  converse.  Thus  while,  through- 
out the  Vertebrata,  the  alimentary  system,  with  the  exception 
of  its  two  extremities,  is  asymmetrically  arranged,  the  re- 
spiratory system,  which  occupies  one  end  of  the  body,  ge- 
nerally deviates  but  little  from  bilateral  symmetry,  and  the 
reproductive  system,  partly  occupying  the  other  end  of  the 
body,  is  in  the  main  bilaterally  symmetrical :  such  deviation 
from  bilateral  symmetry  as  occurs,  being  found  in  its  most 
interiorly-placed  parts,  the  ovaries.  Just  indicating  these 
facts  as  having  a  certain  significance,  it  will  be  best  to  leave 
this  part  of  the  subject  as  too  involved  for  detailed  treat- 
ment. 

Internal  structures  of  one  class,  however,  not  included 
among  the  viscera,  admit  of  general  interpretation — struc- 
tures which,  though  internal,  are  brought  into  tolerably- 
direct  relations  with  the  environing  forces,  and  are  therefore 
subordinate  in  their  forms  to  the  distribution  of  those  forces. 
These  internal  structures  it  will  be  desirable  to  deal  with 
at  some  length  ;  both  because  they  furnish  important  illustra- 
tions enforcing  the  general  argument,  and  because  an  inter- 
pretation of  them  which  we  have  seen  reason  to  reject, 
cannot  be  rejected  without  raising  the  demand  for  some  other 
interpretation. 

V>i..  IT.  9 


CHAPTER  XV. 

THE  SHAPES  OF  VERTEBRATE  SKELETONS. 

§  254.  WHEN  an  elongated  mass  of  any  substance  is 
transversely  strained,  different  parts  of  the  mass  are  ex- 
posed to  forces  of  opposite  kinds.  If,  for  example,  a  bar 
of  metal  or  wood  is  supported  at  its  two  ends,  as  shown  in 
Fig.  281,  and  has  to  bear  a  weight  on  its  centre,  its  lower 


part  is  thrown  into  a  state  of  tension,  while  its  upper  part  is 
thrown  into  a  state  of  compression.  As  will  be  manifest  to 
any  one  who  has  observed  what  happens  on  breaking  a  stick 
across  his  knee,  the  greatest  degree  of  tension  falls  on  the 
fibres  that  form  the  convex  surface,  while  the  fibres  forming 
the  concave  surface  are  subject  to  the  greatest  degree  of 
compression.  Between  these  extremes  the  fibres  at  different 
depths  are  subject  to  different  forces.  Progressing  upwards 
from  the  under  surface  of  the.  bar  shown  in  Fig.  281,  the 
tension  of  the  fibres  becomes  less  ;  and  progressing  down- 
wards from  the  upper  surface,  the  compression  of  the  fibroa 
becomes  less ;  until,  at  a  certain  distance  bstwoen  th;;  two 
surfaces,  there  is  a  place  at  which  the  fibres  are  neithor  ex- 
tended nor  compressed.  This,  shown  by  the  dottel  UUD  ia 


HIE  SHAPES  OF  VERTEBRATE  SKELETONS.       193 

the  figure,  is  called  in  mechanical  language  the  "neutral 
axis."  It  varies  in  position  with  the  nature  of  the  substance 
strained :  being,  in  common  pine-wood,  at  a  distance  of  about 
five  eighths  of  the  depth  from  the  upper  surface  or  three 
eighths  from  the  under  surface.  Clearly,  if  such  a  piece  of 
wood  instead  of  being  subject  to  a  downward  force  is  secured 
at  its  ends  and  subject  to  an  upward  force,  the  distribution 
of  the  compressions  and  tensions  will  be  reversed,  and  the 
neutral  axis  will  be  nearest  to  the  upper  surface.  Fig.  282 
represents  these  opposite  attitudes  of  the  bar  and  the  changed 


position  of  its  neutral  axis :  the  arrow  indicating  the  direc- 
tion of  the  force  producing  the  upward  bend,  and  the  faint 
dotted  line  a,  showing  the  previous  position  of  the  neutral  axis. 
Between  the  two  neutral  axes  will  be  seen  a  central  space  . 
and  it  is  obvious  that  when  the  bar  has  its  strain  from  time 
to  time  reversed,  the  repeated  changes  of  its  molecular  con- 
dition must  affect  the  central  space  in  a  way  different  from 
that  in  which  they  affect  the  two  outer  spaces.  Fig.  283  is 
a  diagram  conveying  some  idea  of  these  contrasts  in  molecular 
condition.  If  A  B  C  D  be  the  middle  part  of  a  bar  thus 
treated,  while  G  H  and  K  L  are  the  alternating  neutral 
axes ;  then  the  forces  to  which  the  bar  is  in  each  case  subject, 
may  be  readily  shown.  Supposing  the  deflecting  force  to 
be  acting  in  the  direction  of  the  arrow  E,  then  the  tensions 
to  which  the  fibres  between  G  and  F  are  exposed,  will  be 
represented  by  a  series  of  lines  increasing  in  length  as  the 
distance  from  G  increases ;  so  that  the  triangle  G  F  M,  will 
express  the  amount  and  distribution  of  all  the  molecular 
tensions.  But  the  molecular  compressions  throughout  the 
space  from  G  to  E,  must  balance  the  molecular  tensions ; 
and  hence,  if  the  triangle  G  E  N  be  made  equal  to  the  tri- 


IU4  MORPHOLOGICAL    DEVELOPMENT. 

angle  G  F  M,  the  parallel  lines  of  which  it  is  composed  (hero 
dotted  for  tho  sake  of  distinction)  will  express  the  amount 


and  distribution  of  the  compressions  between  E  and  G. 
Similarly,  when  the  deflecting  force  is  in  the  direction  of  the 
arrow  F,  the  compressions  and  tensions  will  be  quantitatively 
symbolized  by  the  triangle  K  F  0,  and  K  E  P.  And 
thus  the  several  spaces  occupied  by  full  lines  and  by  dotted 
lines  and  by  the  two  together,  will  represent  the  different 
actions  to  which  different  parts  of  the  transverse  section  are 
subject  by  alternating  transverse  strains.  Here  then  it  is 
made  manifest  to  the  eye  that  the  central  space  between  G 
and  K,  is  differently  conditioned  from  the  spaces  above  ani 
below  it;  and  that  the  difference  of  condition  is  sharply 
marked  off.  The  fibres  forming  the  outer  surface  C  D,  arc 
subject  to  violent  tensions  and  violent  compressions.  Pro- 
gressing inwards  the  tensions  and  compressions  decrease — 
the  tensions  the  more  rapidly.  As  we  approach  the  point  G, 
the  tensions  to  which  the  fibres  are  alternately  subject,  bear 
smaller  and  smaller  ratios  to  the  compressions,  and  disappear 
at  the  point  G.  Thence  to  the  centre  occur  compressions 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.      195 

only,  of  alternating  intensities,  becoming  at  the  centre  small 
and  equal ;  and  from  the  centre  we  advance,  through  a 
reverse  series  of  changes,  to  the  other  side. 

Thus  it  is  demonstrable  that-  any  substance  in  which  the 
power  of  resisting  compression  is  unequal  to  the  power  of 
resisting  tension,  cannot  be  subject  to  alternating  transverse 
strains,  without  having  a  central  portion  differentiated  in  its 
conditions  from  the  outer  portions,  and  consequently  dif- 
ferentiated in  its  structure.  This  conclusion  may  easily  bo 
verified  by  experiment.  If  something  having  a  certain  tough- 
ness but  not  difficult  to  break,  as  a  thick  piece  of  sheet  lead, 
be  bent  from  side  to  side  till  it  is  broken,  the  surface  of  frac- 
ture will  exhibit  an  unlikeness  of  texture  between  the  inner 
and  outer  parts. 

§  255.  And  now  for  the  application  of  this  seemingly-irre- 
levant truth.  Though  it  has  no  obvious  connection  with  the 
interpretation  of  vertebral  structure,  we  shall  soon  see  that  it 
fundamentally  concerns  us. 

The  simplest  type  of  vertebrate  animal,  the  fish,  has  a 
mode  of  locomotion  which  involves  alternating  transverse 
strains.  It  is  not,  indeed,  subjected  to  alternating  transverse 
strains  by  some  outer  agency,  as  in  the  case  we  have  been 
investigating :  it  subjects  itself  to  them.  But  though  the 
strains  are  here  internally  produced  instead  of  externally 
produced,  the  case  is  not  therefore  removed  into  a  wholly 
different  category.  For  sup- 
posing  Fig.  284  to  represent 
the  outline  of  a  fish  when 
bent  on  one  side  (the  dotted  lines  representing  its  outline 
when  the  bend  is  reversed),  it  is  clear  that  part  of  the  sub- 
stance forming  the  convex  half  must  be  in  a  state  of  tension. 
This  state  of  tension  implies  the  existence  in  the  other  half 
of  some  counter-balancing  compression.  And  between  the 
two  there  must  be  a  neutral  axis.  The  way  in  which  this 
conclusion  is  reconcilable  with  the  fact  that  there  is  tension 


196  MORPHOLOGICAL   DEVELOPMENT. 

somewhere  in  the  concave  side  of  a  fish,  since  the  curve  is 
caused  by  muscular  contractions  on  the  concave  side,  will  be 
made  clear  by  the  rude  illustration  which  a  bow  supplies. 
A  bow  may  be  bent  by  a  thrust  against  its  middle  (the  two 
ends  being  held  back),  or  it  may  be  bent  by  contracting 
a  string  that  unites  its  ends ;  but  the  distributions  of  me- 
chanical forces  within  the  wood  of  the  bow,  though  not  quite 
alike  in  the  two  cases,  will  be  very  similar.  Now  while  the 
muscular  action  on  the  concave  side  of  a  fish  differs  from  that 
represented  by  the  tightened  string  of  a  bow,  the  difference 
is  not  such  as  to  destroy  the  applicability  of  the  illustration : 
the  parallel  holds  so  far  as  this,  that  within  that  portion  of 
the  fish's  body  which  is  passively  bent  by  the  contracting 
muscles,  there  must  be,  as  in  a  strung  bow,  a  part  in  com- 
pression, a  part  in  tension,  and  an  intermediate  part  which 
is  neutral. 

Recognizing  the  fact  that  even  in  the  developed  fish  with 
its  complex  locomotive  apparatus,  this  law  of  the  transverse 
strain  holds  in  a  qualified  way,  we  shall  understand  how 
much  more  it  must  hold  in  any  form  that  may  be  supposed 
to  initiate  the  vertebrate  type — a  form  devoid  of  that  seg- 
mentation by  which  the  vertebrate  type  is  more  or  less  cha- 
racterized. We  shall  see  that  assuming  a  rudimentary 
animal  still  simpler  than  the  Amphioxiis,  to  have  a  feeble 
power  of  moving  itself  through  the  water  by  the  undulations 
of  its  body,  or  some  part  of  its  body,  there  will  necessarily 
come  into  play  certain  reactions  that  must  affect  the  median 
portion  of  the  undulating  mass  in  a  way  unlike  that  in 
which  they  affect  its  lateral  portions.  And  if  there  exists  in 
this  median  portion  a  tissue  that  keeps  its  place  with  any 
constancy,  we  may  expect  that  the  differential  conditions 
produced  in  it  by  the  transverse  strain,  will  initiate  a  dif- 
ferentiation. It  is  true  that  the  distribution  of  the  viscera 
in  the  Amphioxus,  Fig.  191,  and  in  the  type  from  which 
we  may  suppose  it  to  arise,  is  such  as  to  interfere  with  this 


THE   SHAPES   OF    VERTEBRATE    SKELETONS.  197 

process.     It  is  also  true  that  the  actions  and  reactions  de- 
scribed would  not  of  themselves  give  to  the  median  portion 


a  cylindrical  shape,  like  that  of  the  cartilaginous  rod  run- 
ning along  the  back  of  the  Amphioxus.  But  what  we  have 
here  to  note  in  the  first  place  is,  that  these  habitual  alternate 
flexions  have  a  tendency  to  mark  off  from  the  outer  parts  an 
unlike  inner  part,  which  may  be  seized  hold  of,  main- 
tained, and  further  modified,  by  natural  selection,  should 
any  advantage  thereby  result.  And  we  have  to  note  in  the 
second  place,  that  an  advantage  is  likely  to  result.  The  con- 
tractions cannot  be  effective  in  producing  undulations,  un- 
less the  general  shape  of  the  body  is  maintained.  External 
muscular  fibres  unopposed  by  an  internal  resistent  mass, 
would  cause  collapse  of  the  body.  To  meet  the  require- 
ments there  must  be  a  means  of  maintaining  longitudinal 
rigidity  without  preventing  bends  from  side  to  side ;  and  such 
a  means  is  presented  by  a  structure  initiated  as  described. 
In  brief,  whether  we  have  or  have  not  the  actual  cause,  we 
have  here  at  any  rate  "a  true  cause."  Though  there  are 
difficulties  in  tracing  out  the  process  in  a  specific  way,  it  may 
at  least  be  said  that  the  mechanical  genesis  of  this  rudiment- 
ary vertebrate  axis  is  quite  conceivable.  And  even  the 
difficulties  may,  I  think,  be  much  more  fully  met  than 
would  at  first  sight  seem  possible. 

AVhat  is  to  be  said  of  the  other  leading  trait  which  the 
simplest  vertebrate  animal  has  in  common  with  all  higher 
vertebrate  animals — the  segmentation  of  its  lateral  va.ua- 


198  MORPHOLOGICAL   DEVELOPMENT. 

cular  masses  ?  Is  this,  too,  explicable  on  the  mechanical 
hypothesis  ?  Have  \ve,  in  the  perpetual  transverse  strains, 
a  cause  for  the  fact  that  while  the  rudimentary  vertebrate 
axis  is  without  any  divisions,  there  are  definite  divisions 
of  the  substance  forming  the  animal's  sides  ?  I  think  we 
have.  A  glance  at  the  distribution  of  forces  under  the 
transverse  strain,  as  represented  in  the  foregoing  diagrams, 
will  show  how  much  more  severe  is  the  strain  on  the  outer 
parts  than  on  the  inner  parts ;  and  how,  consequently,  any 
modifications  of  structure  eventually  necessitated,  will  arise 
peripherally  before  they  arise  centrally.  The  perception  of 
this  may  be  enforced  by  a  simple  experiment.  Take  a  stick 
of  sealing-wax  and  warm  it  slowly  and  moderately  before 
the  fire,  so  as  to  give  it  a  little  flexibility.  Then  bend  it 
gently  until  it  is  curved  into  a  semi-circle.  On  the  convex 
surface  small  cracks  will  be  seen,  and  on  the  concave  sur- 
face wrinkles ;  while  between  the  two  the  substance  remains 
undistorted.  If  the  bend  be  reversed  and  re-reversed,  time 
after  time,  these  cracks  and  wrinkles  will  become  fissures 
which  gradually  deepen.  But  now,  if  changes  of  this  class,  en- 
tailed by  perpetual  transverse  strains,  commence  superficially, 
as  they  manifestly  must ;  there  arise  the  further  questions — 
What  will  be  the  special  modifications  produced  under  these 
special  conditions?  and  through  what  stages  will  these  modifi- 
cations progress  ?  Every  one  has  literally  at  hand  an  example 
of  the  way  in  which  a  flexible  external  layer  that  is  now  ex- 
tended and  now  compressed,  by  the  bending  of  the  mass  it 
covers,  becomes  creased  ;  and  a  glance  at  the  palms  and  the 
fingers  will  show  that  the  creases  are  near  one  another 
where  the  skin  is  thin,  and  far  apart  where  the  skin  is  thick. 
IMween  this  familiar  case  and  the  case  of  the  rhinoceros- 
hide,  in  which  there  are  but  a  few  large  folds,  various  grada- 
tions may  be  traced.  Now  the  like  must  happen  with  the 
increasing  layers  of  contractile  fibres  forming  the  sides  of 
the  muscular  tunic  in  such  a  type  as  that  supposed.  The 
beridings  will  produce  in  them  small  wrinkles  while  they  aro 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.      199 

thin,  but  more  decided  and  comparatively  distant  fissures  aa 
they  become  thick.  Fig  289,  which  is  a 
horizontal  longitudinal  section,  shows 
how  these  thickening  layers  will  adjust 
themselves  on  the  convex  and  the  con- 
cave surfaces,  supposing  the  fibres  of 
which  they  are  composed  to  be  oblique, 
as  their  function  requires ;  and  it  is  not  difficult  to  see  that 
when  once  definite  divisions  have  been  established,  they  will 
advance  inwards  as  the  layers  develop  ;  and  will  so  produce 
a  series  of  muscular  bundles.  Here  then  we  have  something 
like  the  myocommata  which  are  traceable  in  the  Amphioxus, 
and  are  conspicuous  in  all  superior  fishes. 

§  256.  These  speculative  conceptions  I  have  ventured  to 
present  with  the  view  of  showing  that  the  hypothesis  of  the 
mechanical  genesis  of  vertebrate  structure,  is  not  wholly  at 
fault  when  applied  to  the  most  rudimentary  vertebrate  ani- 
mal. Lest  it  should  be  alleged  that  the  question  is  begged 
if  we  set  out  with  a  type  which,  like  the  Amphioxus,  already 
displays  segmentation  throughout  its  muscular  system,  it 
seemed  needful  to  indicate  conceivable  modes  in  which  there 
may  have  been  mechanically  produced  those  leading  traits 
that  distinguish  the  Amph-ioxus.  It  seemed  needful  to 
assign  an  origin  for  the  notochord ;  and  to  this  we  see  a 
clue  in  the  differentiating  effects  of  the  transverse  strain.  It 
seemed  needful  to  account  for  the  existence  of  muscular 
divisions  while  yet  there  are  no  vertebral  divisions ;  and  for 
this,  also,  the  transverse  strain  furnishes  a  feasible  reason. 

But  now,  having  shown  that  the  actions  and  reactions  in- 
volved by  its  mode  of  locomotion,  are  possible  causes  of  those 
rudimentary  structures  which  the  simplest  vertebrate  animal 
presents,  let  us  return  to  the  region  of  established  fact,  and 
consider  whether  such  actions  and  reactions  as  we  actually 
witness,  are  adequate  causes  of  those  observed  differentiations 
and  integrations  which  distinguish  the  more- 'lev  si  oped  vcr- 


200  MJRPH3 LOGICAL    DEVELOPMENT. 

tebrate  animals.  Let  us  see  whether  the  theory  of  mechani- 
cal genesis  afford  us  a  deductive  interpretation  of  the  in- 
ductive generalizations. 

Before  proceeding,  we  must  note  a  process  of  functional 
adaptation  which  here  co-operates  with  natural  selection.  I 
refer  to  the  habitual  formation  of  denser  tissues  at  those 
parts  of  an  organism  which  are  exposed  to  the  greatest 
strains — either  compressions  or  tensions.  Instances  of  hard- 
ening under  compression  are  made  familiar  to  us  by  the 
skin.  We  have  the  general  contrast  between  the  soft  skin 
covering  the  body  at  large,  and  the  indurated  skin  covering 
the  inner  surfaces  of  the  hands  and  the  soles  of  the  feet. 
We  have  the  fact  that  even  within  these  areas  the  parts  on 
which  the  pressure  is  habitually  greatest,  have  the  skin 
habitually  thickest ;  and  that  in  each  person  special  points 
exposed  to  special  pressures  become  specially  dense — often 
as  dense  as  horn.  Further,  we  have  the  converse  fact,  that 
the  skin  of  little-used  hands  becomes  abnormally  thin — even 
losing,  in  places,  that  ribbed  structure  which  distinguishes 
skin  subject  to  rough  usage.  Of  increased  density  directly 
following  increased  tension,  the  skeletons,  whether  of  men 
or  animals,  furnish  abundant  evidence.  Anatomists  easily 
discriminate  between  the  bones  of  a  strong  man  and  those  of 
a  weak  man,  by  the  greater  development  of  those  ridges  and 
crests  to  which  the  muscles  are  attached  ;  and  naturalists,  on 
comparing  the  remains  of  domesticated  animals  with  those 
of  wild  animals  of  the  same  species,  find  kindred  differences. 
The  first  of  these  facts  shows  unmistakably  the  immediate 
effect  of  function  on  structure,  and  by  obvious  alliance  with 
it  the  second  may  be  held  to  do  the  same — both  implying 
that  the  deposit  of  dense  substance  capable  of  great  resist- 
ance, habitually  takes  place  at  points  where  the  tension  is 
excessive. 

Taking  into  account,  then,  this  adaptive  process,  con- 
tinually aided  by  the  survival  of  individuals  in  which  it 
has  taken  place  most  rapidly,  we  may  expect,  on  tracing  up 


THE  SHAPES  O*  VERTEBRATE  SKELETONS.       201 

the  evolution,  of  the  vertebrate  axis,  to  find  that  as  the  mus- 
cular power  becomes  greater  there  arise  larger  and  harder 
masses  of  tissue,  serving  the  muscles  as  points  (Tappui ;  and 
that  these  arise  first  in  those  places  where  the  strains  are 
greatest.  Now  this  is  just  what  we  do  find.  The  myocom- 
mata  are  so  placed  that  their  actions  are  likely  to  afiect  first 
that  upper  coat  of  the  notochord,  where  there  are  found 
"  quadrate  masses  of  somewhat  denser  tissue,"  which  "  seem 
faintly  to  represent  neural  spines,"  even  in  the  Amphioxus. 
It  is  by  the  development  of  the  neural  spines,  and  after  them 
of  the  heomal  spines,  that  the  segments  of  the  vertebral 
column  are  first  marked  out ;  and  under  the  increasing  strain 
of  more- developed  myocommata,  it  is  just  these  peripheral 
appendages  of  the  vertebral  segments  that  must  be  most 
subject  to  the  forces  which  cause  the  formation  of  denser 
tissue.  It  follows  from  the  mechanical  hypothesis  that  as 
the  muscular  segmentation  must  begin  externally  and  pro- 
gress inwards,  so,  too,  must  the  vertebral  segmentation. 
Besides  thus  finding  reason  for  the  fact  that  in  fishes  with 
wholly  cartilaginous  skeletons,  the  vertebral  segments  are 
indicated  by  these  processes,  while  yet  the  notochord  is  un- 
scgmented ;  we  find  a  like  reason  for  the  fact  that  the  tran- 
sition from  the  less-dense  cartilaginous  skeleton  to  the  more- 
dense  osseous  skeleton,  pursues  a  parallel  course.  In  the 
existing  Lcpidosu'cn,  which  by  uniting  certain  piscine  and 
amphibian  characters  betrays  its  close  alliance  with  primitive 
types,  the  axial  part  of  the  vertebral  column  is  unossified, 
\vnile  there  is  ossification  of  the  peripheral  parts.  Similarly 
with  numerous  genera  of  fishes  classed  as  palaeozoic.  The 
fossil  remains  of  them  show  that  while  the  neural  and  haemal 
spines  consisted  of  bone,  the  central  parts  of  the  vertebras 
were  not  bony.  It  may  in  some  cases  be  noted,  too,  both  in 
extant  and  in  fossil  forms,  that  while  the  ossification  is  com- 
plete at  the  outer  extremities  of  the  spines  it  is  incomplete 
at  their  inner  extremities — thus  similarly  implying  centri- 
petal development. 


202  MORPHOLOGICAL   DEVELOPMENT. 

§  257.  After  these  explanations  the  process  of  eventual 
segmentation  in  the  spinal  axis  itself,  will  be  readily  under- 
stood. The  original  cartilaginous  rod  has  to  maintain  longi- 
tudinal rigidity  while  permitting  lateral  flexion.  As  fast  as 
it  becomes  definitely  marked  out,  it  will  begin  to  concentrate 
within  itself  a  great  part  of  those  pressures  and  tensions 
caused  by  transverse  strains.  As  already  said,  it  must  be 
acted  upon  much  in  the  same  manner  as  a  bow,  though  it  is 
bent  by  forces  acting  in  a  more  indirect  way ;  and  Like  a  bow, 
it  must,  at  each  bend,  have  the  substance  of  its  convex  side 
extended  and  the  substance  of  its  concave  side  compressed. 
So  long  as  the  vertebrate  animal  is  small  or  inert,  such  a 
cartilaginous  rod  may  have  sufficient  strength  to  withstand 
the  muscular  strains  ;  but,  other  things  equal,  the  evolution 
of  an  animal  that  is  large,  or  active,  or  both,  implies  mus- 
cular strains  that  must  tend  to  cause  modification  in  such  a 
cartilaginous  rod.  The  results  of  greater  bulk  and  of  greater 
vivacity  may  be  best  dealt  with  separately.  As  the 

animal  increases  in  size,  the  rod  will  grow  both  longer  and 
thicker.  On  looking  back  at  the  diagrams  of  forces  caused 
by  transverse  strains,  it  will  be  seen  that  as  the  rod  grows 
thicker,  its  outer  parts  must  be  exposed  to  more  severe  ten- 
sions and  pressures,  if  the  degree  of  bend  is  the  same.  It  is 
doubtless  true  that  when  the  fish  or  reptile,  advancing  by 
lateral  undulations,  becomes  longer,  the  curvature  assumed 
by  the  body  at  each  movement  becomes  less  ;  and  that  from 
this  cause  the  outer  parts  of  the  notochord  are,  other  things 
equal,  less  strained — the  two  changes  thus  partially  neutral- 
izing one  another.  But  other  things  are  not  equal.  For 
while,  supposing  the  shape  of  the  body  to  remain  con- 
stant, the  force  exerted  in  moving  the  body  increases  as  the 
cubes  of  its  dimensions,  the  sectional  area  of  the  notochord, 
on  which  fall  the  reactions  of  this  exerted  force,  increases 
only  as  the  squares  of  the  dimensions :  whence  results  an 
iutenser  stress  upon  its  substance.  Merely  noting  that  the 
other  varying  factor — the  resistance  of  the  water — may  here 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.       203 

be  left  out  of  the  account  (since  for  similar  masses  moving 
with  equal  velocities  the  resistances  increase  but  little  faster 
than  the  squares  of  the  dimensions,  which  is  the  rate  at  which 
the  sectional  areas  of  the  notochords  increase)  we  see  that  aug- 
menting bulk,  taken  alone,  involves  but  a  moderate  residuary 
increase  of  strain  on  each  portion  of  the  notochord;  and 
this  is  probably  the  reason  why  it  is  possible  for  a  large  slug- 
(jish  fish  like  the  Sturgeon,  to  retain  the  notochordal  struc- 
ture. But  now,  passing  to  the  effects  of  greater  ac- 
tivity, a  like  dynamical  inquiry  at  once  shows  us  how  rapidly 
the  violence  of  the  actions  and  reactions  rises  as  the  move- 
ments become  more  vivacious.  In  the  first  place,  the  resist- 
ance of  a  medium  such  as  water  increases  as  the  square  of 
the  velocity  of  the  body  moving  through  it ;  so  that  to  main- 
tain double  the  speed,  a  fish  has  to  expend  four  times  tho 
energy.  But  the  fish  has  to  do  more  than  this — it  has  to 
initiate  this  speed,  or  to  impress  on  its  mass  the  force  implied 
by  this  speed.  Now  the  vis  viva  of  a  moving  body  varies  as 
the  square  of  the  velocity ;  whence  it  follows  that  the  energy 
required  to  generate  that  vis  viva  is  measured  by  the  square 
of  the  velocity  it  produces.  Consequently,  did  the  fish  put 
itself  in  motion  instantaneously,  the  expenditure  of  energy  in 
generating  its  own  vis  viva  and  simultaneously  overcoming 
the  resistance  of  the  water,  would  vary  as  the  fourth  power 
of  the  velocity.  But  the  fish  cannot  put  itself  in  motion 
instantaneously — it  must  do  it  by  increments ;  and  thus  it 
results  that  the  amounts  of  the  forces  expended  to  give  itself 
different  velocities  must  be  represented  by  some  series  of 
numbers  falling  between  the  squares  and  the  fourth  powers 
of  those  velocities.  Were  the  increments  slowly  accumulated, 
the  ratio  of  increasing  effort  would  but  little  exceed  the  ratio 
of  the  squares ;  but  whoever  observes  the  sudden,  convulsive 
action  with  which  an  alarmed  fish  darts  out  of  a  shallow  into 
deep  water,  will  see  that  the  velocity  is  very  rapidly  gener- 
ated, and  that  therefore  the  ratio  of  increasing  effort  probably 
exceeds  the  ratio  of  the  squares  very  considerably.  At  any 


204  MORPHOLOGICAL   DEVELOPMENT. 

rate  it  will  be  clear  that  the  efforts  made  by  fish  in  rushing 
upon  prey  or  escaping  enemies  (and  it  is  these  extreme  efforts 
which  here  concern  us)  must,  as  fish  become  more  active, 
rapidly  exalt  the  strains  to  be  borne  by  their  motor  organs  ; 
and  that  of  these  strains,  those  which  fall  upon  the  noto- 
chord  must  be  exalted  in  proportion  to  the  rest.  Thus  the 
development  of  locomotive  power,  which  survival  of  the 
fittest  must  tend  in  most  cases  to  favour,  involves  such  in- 
crease of  stress  on  the  primitive  cartilaginous  rod  as  will 
tend,  other  things  equal,  to  cause  its  modification. 

What  must  its  modification  be  ?  Considering  the  compli- 
cation of  the  influences  at  work,  conspiring,  as  above  indi- 
cated, in  various  ways  and  degrees,  we  cannot  expect  to  do 
more  than  form  an  idea  of  its  average  character.  The  nature 
of  the  changes  which  the  notochord  is  likely  to  undergo,  where 
greater  bulk  is  accompanied  by  higher  activity,  is  rudely 
indicated  by  Figs.  291,  292,  and  293.  The  successively 


thicker  lines  represent  the  successively  greater  strains  to 
which  the  outer  layers  of  tissue  are  exposed ;  and  the  widen- 
ing inter-spaces  represent  the  greater  extensions  which  they 
have  to  bear  when  they  become  convex,  or  else  the  greater 
gaps  that  must  be  formed  in  them.  Had  these  outer  layers 
to  undergo  extension  only,  as  on  the  convex  side,  continued 
natural  selection  might  result  in  the  formation  of  a  tissue 
elastic  enough  to  admit  of  the  requisite  stretching.  But  at 
each  alternate  bend,  these  outer  layers,  becoming  concave, 
are  subject  to  increased  compression — a  compression  which 
they  cannot  withstand  if  they  have  become  simply  more 
extensible.  To  withstand  this  greater  compression  they  must 
become  harder  as  well  as  more  extensible.  How  are 
these  two,  requirements  to  be  reconciled  ?  If,  as  facts  war- 
rant us  in  supposing,  a  formation  of  denser  substance  occurs 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.      205 

at  those  parts  of  the  notochord  where  the  strain  is  greatest ; 
it  is  clear  that  this  formation  cannot  so  go  on  as  to  produce 
a  continuous  mass  :  the  perpetual  flexions  must  prevent  this. 
If  matter  that  will  not  yield  at  each  bend,  is  deposited  while 
the  bendings  are  continually  taking  place,  the  bendings  will 
maintain  certain  places  of  discontinuity  in  the  deposit — 
places  ai  which  the  whole  of  the  stretching  consequent  on 
each  bend  will  be  concentrated.  And  thus  the  tendency  will 
be  to  form  segments  of  hard  tissue  capable  of  great  resistance 
to  compression,  with  intervals  filled  by  elastic  tissue  capable 
of  great  resistance  to  extension — a  vertebral  column. 

And  now  observe  how  the  progress  of  ossification  is  just 
such  as  conforms  to  this  view.  That  centripetal  develop- 
ment of  segments  which  holds  of  the  vertebrate  animal  as  a 
whole,  as,  if  caused  by  transverse  strains,  it  ought  to  do,  and 
which  holds  of  the  vertebral  column  as  a  whole,  as  it  ought 
to  do,  holds  also  of  the  central  axis.  On  the  mechanical 
hypothesis,  the  outer  surface  of  the  notochord  should  be  the 
first  part  to  undergo  induration,  and  that  division  into  seg- 
ments that  must  accompany  induration.  And  accordingly, 
in  a  vertebral  column  of  which  the  axis  is  beginning  to 
ossify,  the  centrums  consist  of  bony  rings  inclosing  a  still 
continuous  rod  of  cartilage. 

§  258.  Sundry  other  general  facts  which  the  comparative 
morphology  of  the  Vertcbrata  discloses,  supply  further  con- 
firmation. Let  us  take  first  the  structure  of  the  skull. 

On  considering  the  arrangement  of  the  muscular  flakes,  or 
myocommata,  in  any  ordinary  fish  that  comes  to  table — an 
arrangement  already  sketched  out  in  the  Amphioxus — it  is  not 
difficult  to  see  that  that  portion  of  the  body  out  of  which  the 
head  of  the  vertebrate  animal  becomes  developed,  is  a  por- 
tion which  cannot  subject  itself  to  bendings  in  the  same 
degree  as  the  rest  of  the  body.  The  muscles  developed  there 
must  be  comparatively  short,  and  much  interfered  with  by 
the  pre-existing  orifices.  Hence  the  cephalic  part  will  not 


206  MORPHOLOGICAL    DEVELOPMENT. 

partake  in  any  considerable  degree  of  the  lateral  undula« 
tions ;  and  there  will  not  tend  to  arise  in  it  any  such  distinct 
segmentation  as  arises  elsewhere.  We  have  here,  then,  an 
explanation  of  the  fact,  that  from  the  beginning  the  develop- 
ment of  the  head  follows  a  course  unlike  that  of  the  spinal 
column ;  and  of  the  fact  that  the  segmentation,  so  far  as  it 
can  be  traced  in  the  head,  is  most  readily  to  be  traced  in  the 
occipital  region  and  becomes  lost  in  the  region  of  the  face. 
Still  more  significantly,  we  have  an  explanation  of  the  fact 
that  the  base  of  the  skull,  answering  to  the  front  end  of  the 
notochord,  never  betrays  any  sign  of  segmentation.  This, 
which  is  absolutely  at  variance  with  the  hypothesis  of  the 
transcendental  anatomists,  is  in  complete  harmony  with  tho 
foregoing  hypothesis.  For  if,  as  we  have  seen,  the  segmenta- 
tion consequent  on  mechanical  actions  and  reactions  must 
progress  from  without  inwards,  affecting  last  of  all  the  axis ; 
and  if,  as  we  have  seen,  the  region  of  the  head  ia  so  circum- 
stanced that  the  causes  of  segmentation  act  but  feebly  even 
on  its  periphery  ;  then,  it  is  to  be  expected  that  its  axis 
will  not  be  segmented  at  all :  that  portion  of  the  primitive 
notochord  which  is  included  in  the  head,  having  to  un- 
dergo no  lateral  bendings,  may  ossify  without  division  into 
segments. 

Of  other  incidental  evidences  supplied  by  comparative 
morphology,  let  me  next  refer  to  the  supernumerary  bones, 
which  the  theory  of  Goethe  and  Oken  as  elaborated  by  Prof. 
Owen,  has  to  get  rid  of  by  gratuitous  suppositions.  In  many 
fishes,  for  example,  there  are  what  have  been  called  inter- 
neural  spines  and  inter-haemal  spines.  These  cannot  by  any 
ingenuity  be  affiliated  upon  the  archetypal  vertebra,  and  they 
are  therefore  arbitrarily  rejected  as  bones  belonging  to  the 
exo-skeleton ;  though  in  shape  and  texture  they  are  similar 
to  the  spines  between  which  they  are  placed.  On  the  hy- 
pothesis of  evolution,  however,  these  additional  bones  are 
accounted  for  as  arising  under  actions  like  those  that  gave 
origin  to  the  bones  adjacent  to  them.  And  similarly  with 


THE  SHAPES  OF  VERTEBRATE  SKELETONS       207 

such,  bones  as  those  called  sesamoid ;   together  with  others 
too  numerous  to  name. 

Again,  in  the  course  of  evolution,  both  as  displayed  m  the 
Vertebra  ta  generally  and  in  each  vertebrate  embryo,  three 
skeletons  succeed  one  another — the  membranous,  the  car- 
tilaginous, and  the  osseous.  These  substitutions  take  place 
variously  and  unsystematically.  While  one  part  of  a  skele- 
ton retains  the  membranous  character,  another  part  of  the 
same  skeleton  has  become  cartilaginous.  At  the  same  time 
that  certain  components  have  become  partially  or  completely 
ossified,  other  components  continue  cartilaginous  or  mem- 
branous. Further,  though  there  is  a  general  succession  of 
these  stages,  the  succession  is  not  regularly  maintained ;  for 
in  many  cases  bones  are  formed  by  the  deposit  of  osseous 
matter  in  portions  of  the  membranous  skeleton,  which  thus 
do  not  pass  through  the  cartilaginous  stage.  "  Nor,"  says 
Prof.  Huxley,  "does  any  one  of  these  states  ever  completely 
obliterate  its  predecessor ;  more  or  less  cartilage  and  mem- 
brane entering  into  the  composition  of  the  most  completely 
ossified  skull,  and  more  or  less  membrane  being  discoverable 
in  the  most  completely  chondrified  skull."  And  then,  too, 
the  processes  of  chondrification  and  ossification  often  proceed 
with  but  little  respect  for  the  pre-existing  divisions;  but 
severally  may  result  in  the  establishment  of  two  parts  where 
there  was  before  one,  or  one  where  there  were  before  two. 
Now  wholly  incongruous  as  these  facts  are  with  the  hypothe- 
sis of  an  archetypal  skeleton,  they  are  quite  congruous  with 
the  mechanical  hypothesis.  This  shows  us  why,  in  the 
course  of  evolution,  a  feebly-resisting  membranous  structure 
came  to  be  replaced  by  a  more-resisting  cartilaginous  struc- 
ture, and  this,  again,  by  a  still-more-resisting  osseous  struc- 
ture ;  and  why.  therefore,  these  successive  stages  succeed  one 
another,  as  it  seems  so  superfluously,  in  the  vertebrate  em- 
bryo. And  it  further  shows  us  why  there  is  irregularity  in 
the  succession ;  seeing  that  the  varying  mechanical  ac- 
tions to  which  the  varying  habits  of  the  Vertcbrata  have 


208  MORPHOLOGICAL    DEVELOPMENT. 

exposed  them,  have  involved  variations  in  the  process  of 
solidification. 

§  259.  Of  course  the  foregoing  synthesis  is  to  be  taken 
simply  as  an  adumbration  of  the  process  by  which  the  verte- 
brate structure  may  have  arisen  through  the  continued  actions 
of  known  agencies.  The  motive  for  attempting  it  has  been 
two-fold.  Having,  as  before  said,  given  reasons  for  con- 
cluding that  the  segments  of  a  vertebrate  animal  are  not 
homologous  in  the  same  sense  as  those  of  an  annulose  animal 
or  a  phaenogamic  axis,  it  seemed  needful  to  do  something 
towards  showing  how  they  are  otherwise  to  be  accounted  for ; 
and  having  here,  for  our  general  subject,  the  likenesses  and 
differences  among  the  parts  of  organisms,  as  determined  by 
incident  forces,  it  seemed  out  of  the  question  to  pass  by  the 
problem  presented  by  the  vertebrate  skeleton. 

Leaving  out  all  that  is  hypothetical,  the  general  argument 
may  be  briefly  presented  thus : — The  evolution  from  the 
simplest  known  vertebrate  animal,  of  a  powerful  and  active 
vertebrate  animal,  implies  the  development  of  a  stronger 
internal  fulcrum.  The  internal  fulcrum  cannot  be  made 
stronger  without  becoming  more  dense.  And  it  cannot  be- 
come more  dense  while  retaining  its  lateral  flexibility,  with- 
out becoming  divided  into  segments.  Further,  in  conformity 
with  the  general  principles  thus  far  traced,  these  segments 
must  be  alike  in  proportion  as  the  forces  to  which  they  are 
exposed  are  alike,  and  unlike  in  proportion  as  these  forces 
are  unlike ;  and  so  there  necessarily  results  that  unity  in 
variety  by  which  the  vertebral  column  is  from  the  beginning 
characterized.  Once  more,  we  see  that  the  explanation  ex- 
tends to  those  innumerable  and  more-marked  divergences 
from  homogeneity,  which  vertebrae  undergo  in  the  various 
higher  animals.  Thus,  the  production  of  vertebra?,  the  pro- 
duction of  likenesses  among  vertebrae,  and  the  production  of 
unlikenesses  among  vertebrae,  are  interpretable  as  parts  of 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.       209 

one  general  process,  and  as  harmonizing  with  one  general 
principle. 

Whether  sufficient  or  insufficient,  the  explanation  here 
given  assigns  causes  of  known  kinds  producing  effects  such 
as  they  are  known  to  produce.  It  does  not,  as  a  solution  of 
one  mystery,  offer  another  mystery  of  which  no  solution  is 
to  be  asked.  It  does  not  allege  a  Platonic  t'Sc'a,  or  fictitious 
entity,  which  explains  the  vertebrate  skeleton  by  absorbing 
into  itself  all  the  inexpiicability.  On  the  contrary,  it  assumes 
nothing  beyond  agencies  by  which  structures  in  general  are 
moulded — agencies  by  which  these  particular  structures  are, 
indeed,  notoriously  modifiable.  An  ascertained  cause  of 
certain  traits  in  vertebrao  and  other  bones,  it  extends  to  all 
other  traits  of  vertebra? ;  and  at  the  time  assimilates  the 
morphological  phenomena  they  present  to  much  wider  classes 
of  morphological  phenomena. 


CHAPTER  XVI. 

THE   SHAPES   OF   ANIMAL    CELLS. 

§  260.  AMONG  animals  as  among  plants,  the  laws  of  mor- 
phological differentiation  must  be  conformed  to  by  the  mor- 
phological units,  as  well  as  by  the  larger  parts  and  the  wholes 
formed  of  them.  It  remains  here  to  point  out  that  the  con- 
formity is  traceable  where  the  conditions  are  simple. 

In  the  shapes  assumed  by  those  rapidly-multiplying  cells 
out  of  which  each  animal  is  developed,  there  is  a  conspicuous 
subordination  to  the  surrounding  actions. 
Fig.  294  represents  the  cellular  embryonic 
mass  that  arises  by  repeated  spontaneous 
fissions.  In  it  we  see  how  the  cells,  origin- 
ally spherical,  are  changed  by  pressure 
against  one  another  and  against  the  limit- 
ing membrane ;  and  how  their  likenesses 
and  unlikenesses  are  determined  by  the  likenesses  and  un- 
likenesses  of  the  forces  to  which  they  are  exposed.  This  fact 
may  be  thought  scarcely  worth  pointing  out.  But  it  is 
worth  pointing  out,  because  what  is  here  so  obvious  a  con- 
sequence of  mechanical  actions,  is  in  other  cases  a  conse- 
quence of  actions  composite  in  their  kinds  and  involved  in 
their  distribution.  Just  as  the  equalities  and  inequalities  of 
dimensions  among  aggregated  cells,  are  here  caused  by  the 
equalities  and  inequalities  among  their  mutual  pressures  in 
different  directions ;  so,  though  less  manifestly,  the  equalities 


THE  SHAPES  OF  ANIMAL  CELLS. 


211 


and  inequalities  of  dimensions  among  other  aggregated  cells, 
aro  caused  by  the  equalities  and  inequalities  of  the  osmotic, 
chemical,  thermal,  and  other  forces  besides  the  mechanical, 
to  which  their  different  positions  subject  them. 

§  261.  This  we  shall  readily  see  on  observing  the  or- 
dinary structures  of  limiting  membranes  internal  and  ex- 
ternal. In  Fig.  295,  is  2SS 
shown  a  much-magnified 
section  of  a  papilla  from 
the  gum.  The  cells  of 
which  it  is  composed 
originate  in  its  deeper 
part;  and  are  at  first 
approximately  spherical. 
Those  of  them  which,  as  they  develop,  are  thrust  outwards  by 
the  new  cells  that  continually  take  their  places,  have  their 
shapes  gradually  changed.  As  they  grow  and  successively 
advance  to  replace  the  superficial  cells,  when  these  exfoliate, 
they  become  exposed  to  forces  that  are  more  and  more  dif- 
ferent in  the  direction  of  the  surface  from  what  they  are  in 
lateral  directions  ;  and  their  dimensions  gradually  assume 
corresponding  differences. 

Another  species  of  limiting  membrane,  called  cylinder- 
epithelium,  is  represented 
in  Fig.  296.  Though  its 
mode  of  development  is 
such  as  to  render  the 
shapes  of  its  cells  quite 
unlike  those  of  pavement- 
epithelium,  as  the  above-described  kind  is  sometimes  called, 
its  cells  equally  exemplify  the  same  general  truth.  For  the 
chief  contrast  which  each  of  them  presents,  is  the  contrast 
between  its  dimension  at  right  angles  to  the  surface  of  the 
membrane,  and  its  dimension  parallel  to  that  surface. 

It  is  needless  for  our  present  purpose  to  examine  further 


212  MORPHOLOGICAL   DEVELOPMENT. 

the  evidence  furnished  by  Histology ;  nor,  indeed,  would 
further  examination  of  this  evidence  be  likely  to  yield  de- 
finite results.  In  the  cases  given  above  we  have  marked 
differences  among  the  incident  forces ;  and  therefore  have  a 
chance  of  finding,  as  we  do  find,  relations  between  these  and 
differences  of  form.  But  the  cells  composing  masses  of 
tissue  are  severally  subject  to  forces  that  are  indeterminate  ; 
and  therefore  the  interpretation  of  their  shapes  is  imprac- 
ticable. It  must  suffice  that  so  far  as  the  facts  go  they  are 
congruous  with  the  hypothesis. 


CHAPTER  XVII. 

8UMMABT   OF   MORPHOLOGICAL   DEVELOPMENT. 

£  262.  THAT  any  formula  should  be  capable  of  expressing 
a  common  character  in  the  shapes  of  things  so  unlike  as  a 
tree  and  a  cow,  a  flower  and  a  centipede,  is  a  remarkable 
fact ;  and  is  a  fact  which  affords  strong  prima  facie  evidence 
of  truth.  For  in  proportion  to  the  diversity  and  multiplicity 
of  the  cases  to  which  any  statement  applies,  is  the  probability 
that  it  sets  forth  the  essential  relations.  Those  connexions 
wliich  remain  constant  under  all  varieties  of  manifestation, 
are  most  likely  to  be  the  causal  connexions. 

Still  higher  will  appear  the  likelihood  of  an  alleged  law  of 
organic  form  possessing  so  great  a  comprehensiveness,  when 
we  remember  that  on  the  hypothesis  of  Evolution,  there  must 
exist  between  all  organisms  and  their  environments,  certain 
congruities  expressible  in  terms  of  their  actions  and  reac- 
tions. The  forces  being,  on  this  hypothesis,  the  causes  of  the 
forms,  it  is  inferable,  a  priori,  that  the  forms  must  admit  of 
generalization  in  terms  of  the  forces ;  and  hence,  such  a 
generalization  arrived  at  a  posteriori,  gains  the  further  pro- 
bability due  to  fulfilment  of  anticipation. 

Nearer  yet  to  certainty  seems  the  conclusion  thus  reached, 
on  finding  that  it  does  but  assert  in  their  special  manifesta- 
tions, the  laws  of  Evolution  in  general — the  laws  of  that  uni- 
versal re-distribution  of  rc  atter  and  motion  which  hold  through- 
out the  totality  of  things,  as  well  as  in  each  of  its  parts. 


214  MORPHOLOGICAL    DEVELOPMENT. 

It  will  be  useful  to  glance  back  over  the  various  minor 
inferences  arrived  at,  and  contemplate  them  in  their  ensem- 
ble from  these  higher  points  of  view. 

§  263.  That  process  of  integration  which  every  plant  dis- 
plays during  its  life,  we  found  reason  to  think  has  gone  on 
during  the  life  of  the  vegetal  kingdom  as  a  whole.  Proto- 
plasm into  cells,  cells  into  folia,  folia  into  axes,  axes  into 
branched  combinations — such,  in  brief,  are  the  stages  passed 
through  Ity  every  shrub ;  and  such  appear  to  have  been  the 
stages  through  which  plants  of  successively-higher  kinds 
have  been  evolved  from  lower  kinds.  Even  among  certain 
groups  of  plants  now  existing,  we  find  aggregates  of  the  first 
order  passing  through  various  gradations  into  aggregates  of 
the  second  order — here  fonning  small,  incoherent,  indefinite 
assemblages,  and  there  forming  large,  definite,  coherent 
fronds.  Similar  transitions  are  traceable  through  which 
these  integrated  aggregates  of  the  second  order  pass  into 
aggregates  of  the  third  order :  in  one  species  the  unions  of 
parent-fronds  with  the  fronds  that  bud  out  from  them,  being 
temporary,  and  in  another  species  such  unions  being  longer 
continued ;  until,  in  species  still  higher,  by  a  gemmation 
that  is  habitual  and  regular,  there  is  produced  a  definitely- 
integrated  aggregate  of  the  third  order — an  axis  bearing 
fronds  or  leaves.  And  even  between  this  type  and  a  type 
further  compounded,  a  link  occurs  in  the  plants  which  cast 
off,  in  the  shape  of  bulbils,  some  of  the  young  axes  they 
produce.  As  among  plants,  so  among  animals.  A 

like  spontaneous  fission  of  cells  ends  here  in  separation,  there 
in  partial  aggregation,  while  elsewhere,  by  closer  combina- 
tion of  the  multiplying  units,  there  arises  a  coherent  and 
tolerably  definite  individual  of  the  second  order.  By  the 
budding  of  individuals  of  the  second  order,  there  are  in  some 
cases  produced  other  separate  individuals  like  them ;  in  some 
cases  temporary  aggregates  of  such  like  individuals ;  and  in 
other  cases  permanent  aggregates  of  them  :  certain  of  which 


SUMMARY    OP    MORPHOLOGICAL    DEVELOPMENT.  216 

become  so  definitely  integrated  that  the  individualities  of 
their  component  members  are  almost  lost  in  a  tertiary  indi- 
viduality. 

Along  with  this  progressive  integration  there  has  gone  on 
a  progressive  differentiation.  Vegetal  units  of  whatever 
order,  originally  homogeneous,  have  become  heterogeneous 
while  they  have  become  united.  Spherical  cells  aggregating 
into  threads,  into  laminae,  into  masses,  and  into  special  tis- 
sues, lose  their  sphericity  ;  and  instead  of  remaining  all 
alike  assume  innumerable  unlikenesses — from  uniformity  pass 
into  multiformity.  Fronds  combining  to  form  axes,  severally 
acquire  definite  differences  between  their  attached  ends  and 
their  free  ends ;  while  they  also  diverge  from  one  another 
in  their  shapes  at  different  parts  of  the  axes  they  compose. 
And  axes,  uniting  into  aggregates  of  a  still  higher  order,  be- 
come more  or  less  contrasted  in  their  sizes,  curvatures,  and 
arrangements  of  their  appendages.  Similarly  among 

animals.  Those  components  of  them  which,  with  a  certain 
license,  we  class  as  morphological  units,  while  losing  their 
minor  individualities  in  the  major  individualities  formed  of 
them,  grow  definitely  unlike  as  they  grow  definitely  com- 
bined. And  where  the  aggregates  so  produced  become,  by 
coalescence,  segments  of  aggregates  of  a  still  higher  order, 
they,  too,  diverge  from  one  another  in  their  shapes. 

The  morphological  differentiation  which  thus  goes  hand  in 
hand  with  morphological  integration,  is  clearly  what  the 
perpetually-complicating  conditions  would  lead  us  to  antici- 
pate. Every  addition  of  a  new  unit  to  an  aggregate  of  such 
units,  must  affect  the  circumstances  of  the  other  units  in  all 
varieties  of  ways  and  degrees,  according  to  their  relative 
positions — must  alter  the  distribution  of  mechanical  strains 
throughout  the  mass,  must  modify  the  process  of  nutrition, 
must  affect  the  relations  of  neighbouring  parts  to  surround- 
ing diffused  actions ;  that  is,  must  initiate  a  changed  inci- 
dence of  forces  tending  ever  to  produce  changed  structural 
arrangements, 

VOL.  II,  10 


216  MORPHOLOGICAL    DEVELOPMENT. 

§  264.  This  broad  statement  of  the  correspondence  be- 
tween the  general  facts  of  Morphological  Development  and 
the  principles  of  Evolution  at  large,  may  be  reduced  to  state- 
ments of  a  much  more  specific  kind.  The  phenomena  of 
symmetry  and  unsymmetry  and  asymmetry,  which  we  have 
traced  out  among  organic  forms,  are  demonstrably  in  har- 
mony with  those  laws  of  the  re-distribution  of  matter  and 
motion  to  which  Evolution  conforms.  Besides  the  myriad- 
fold  illustrations  of  the  instability  of  the  homogeneous,  that 
are  afforded  by  these  aggregates  of  units  of  each  order,  which, 
at  first  alike,  lapse  gradually  into  unlikeness ;  and  besides 
the  myriad- fold  illustrations  of  the  multiplication  of  effects, 
which  these  ever-complicating  differentiations  exhibit  to  us  ; 
we  have  also  myriad-fold  illustrations  of  the  definite  equal- 
ities and  inequalities  of  structures,  produced  by  definite 
equalities  and  inequalities  of  forces. 

The  proposition  arrived  at  when  dealing  with  the  causes 
of  Evolution,  "  that  in  the  actions  and  reactions  of  force  and 
matter,  an  unlikeness  in  either  of  the  factors  necessitates  an 
unlikeness  in  the  effects ;  and  that  in  the  absence  of  unlike- 
ness in  either  of  the  factors  the  effects  must  be  alike  "  (First 
Principles,  §  129),  is  the  general  formula  including  all 
these  particular  likenesses  and  unlikenesses  of  parts  which 
we  have  been  tracing.  For  have  we  not  everywhere  seen 
that  the  strongest  contrasts  are  between  the  parts  that  are 
most  contrasted  in  their  conditions ;  while  the  most  similar 
parts  are  those  most- similarly  conditioned  ?  In  every  plant 
the  leading  difference  is  between  the  attached  end  and  the 
free  end ;  in  every  branch  it  is  the  same ;  in  every  leaf  it  is 
the  same.  And  in  every  plant  the  leading  likenesses  are 
those  between  the  two  sides  of  the  branch,  the  two  sides  of 
the  leaf,  and  the  two  sides  of  the  flower,  where  these  parts 
are  two-sided  in  their  conditions  ;  or  between  all  sides  of  the 
branch,  all  sides  of  the  leaf,  and  all  sides  of  the  flower,  where 
these  parts  are  similarly  conditioned  on  all  sides.  So,  too,  is 
it  with  animals  that  move  about.  The  most  marked  contrasts 


SUMMARY  OF  MORPHOLOGICAL   DEVELOPMENT.  217 

they  present  are  those  between  the  part  in  advance  and  the 
part  behind,  and  between  the  upper  part  and  the  under  part ; 
while  there  is  complete  correspondence  between  the  two 
sides.  Externally  the  likenesses  and  differences  among 
limbs,  and  internally  the  likenesses  and  differences  among 
vertebrae,  are  expressible  in  terms  of  this  same  law. 

And  here,  indeed,  we  may  see  clearly  that  these  truths  are 
corollaries  from  that  ultimate  truth  to  which  all  phenomena 
of  Evolution  are  referable.  It  is  an  inevitable  deduction 
from  the  persistence  of  force,  that  organic  forms  which  have 
been  progressively  evolved  must  present  just  those  funda- 
mental traits  of  form  which  we  find  them  present.  It  cannot 
but  be  that  during  the  intercourse  between  an  organism  and 
its  environment,  equal  forces  acting  under  equal  conditions 
must  produce  equal  effects  ;  for  to  say  otherwise,  is,  by  im- 
plication, to  say  that  some  force  can  produce  more  or  less 
than  its  equivalent  effect,  which  is  to  deny  the  persistence  of 
force.  Hence  those  parts  of  an  organism  which  are,  by  its 
habits  of  life,  exposed  to  like  amounts  and  like  combinations 
of  actions  and  reactions,  must  develop  alike ;  while  unlike- 
nesses  of  development  must  as  unavoidably  follow  unlike- 
nesses  among  these  agencies.  And  this  being  so,  all  the 
specialities  of  symmetry  and  unsymmetry  and  asymmetry 
which  we  have  traced,  are  necessary  consequences. 


PART   T. 
PHYSIOLOGICAL    DEVELOPMENT. 


CHAPTER  I. 


THE    PROBLEMS    OF    PHYSIOLOGY. 

§  265.  THE  questions  to  be  treated  under  the  above 
title  are  widely  different  from  those  which  it  ordinarily 
expresses.  We  have  no  alternative,  however,  but  to  use 
Physiology  in  a  sense  co-extensive  with  that  in  which  we 
have  used  Morphology.  We  must  here  consider  the  facts  of 
function  in  a  manner  parallel  to  that  in  which  we  have, 
in  the  foregoing  Part,  considered  the  facts  of  structure. 
As,  hitherto,  we  have  concerned  ourselves  with  those  most 
general  phenomena  of  organic  form  which,  holding  irre- 
spective of  class  and  order  and  sub-kingdom,  illustrate  the 
processes  of  integration  and  differentiation  characterizing 
Evolution  in  general ;  so,  now,  we  have  to  concern  ourselves 
with  the  evidences  of  those  differentiations  and  integrations 
of  organic  functions  which  have  simultaneously  arisen,  and 
which  similarly  transcend  the  limits  of.  zoological  and 
botanical  divisions.  How  heterogeneities  of  action  have 
progressed  along  with  heterogeneities  of  structure — that  is 
the  inquiry  before  us ;  and  obviously,  in  pursuing  it,  all 
the  specialities  with  which  Physiology  usually  deals  can 
serve  us  only  as  materials. 

Before  entering  on  the  study  of  Morphological  Develop- 
ment, it  was  pointed  out  that  while  facts  of  structure  may 
be  empirically  generalized  apart  from  facts  of  function,  they 


•^2  PHYSIOLOGICAL   DEVELOPMENT. 

cannot  be  rationally  interpreted  apart ;  and  throughout  the 
foregoing  pages  this  truth  has  been  made  abundantly  mani- 
fest. Here  we  are  obliged  to  recognize  the  inter-dependence 
still  more  distinctly ;  for  the  phenomena  of  function  cannot 
even  be  conceived  without  direct  and  perpetual  consciousness 
of  the  phenomena  of  structure.  Though  the  subject-matter 
of  Physiology  is  as  broadly  distinguished  from  the  subject- 
matter  of  Morphology  as  motion  is  from  matter;  yet,  just 
as  the  laws  of  motion  cannot  be  known  apart  from  some 
matter  moved,  so  there  can  be  no  knowledge  of  function 
without  a  knowledge  of  some  structure  as  performing  func- 
tion. 

Much  more  than  this  is  obvious.  The  study  of  functions, 
considered  from  our  present  point  of  view  as  arising  by 
Kvolution,  must  be  carried  on  mainly  by  the  study  of  the  cor- 
relative structures.  Doubtless,  by  experimenting  on  the  organ- 
isms that  are  growing  and  moving  around  us,  we  may 
ascertain  the  connexions  existing  among  certain  of  their 
actions,  while  we  have  little  or  no  knowledge  of  the  special 
parts  concerned  in  those  actions.  In  a  living  animal  that 
can  be  conveniently  kept  under  observation,  we  may  learn 
the  way  in  which  conspicuous  functions  vary  together — how 
the  rate  of  a  man's  pulse  increases  with  the  amount  of 
muscular  exertion  he  is  undergoing ;  or  how  a  horse's 
rapidity  of  breathing  is  in  part  dependent  on  his  speed. 
But  though  observations  of  this  order  are  indispensable — 
though  by  accumulation  and  comparison  of  such  obser- 
vations we  learn  which  parts  perform  which  functions — 
though  such  observations,  prosecuted  so  as  to  disclose 
the  actions  of  all  parts  under  all  circumstances,  constitute, 
when  properly  generalized  and  co-ordinated,  what  is  com- 
monly understood  as  Physiology ;  yet  such  observations 
help  us  but  a  little  way  towards  learning  how  functions 
came  to  be  established  and  specialized.  We  have  next  to 
no  power  of  tracing  up  the  genesis  of  a  function  considered 
purely  as  a  function — no  opportunity  of  observing  the 


THE    PROBLEMS    OF    PHYSIOLOGY,  22'd 

progressively-increasing  quantities  of  a  given  action  that 
have  arisen  in  any  order  of  organisms.  In  nearly  all  cases 
we  are  able  only  to  establish  the  greater  growth  of  the  part 
which  we  have  found  performs  the  action,  and  to  infer  that 
greater  action  of  the  part  has  accompanied  greater  growth 
of  it.  The  tracing  out  of  Physiological  Development,  then, 
becomes  substantially  a  tracing  out  of  the  development  of 
the  organs  by  which  the  functions  are  known  to  be  dis- 
charged— the  differentiation  and  integration  of  the  functions 
being  presumed  to  have  progressed  hand  in  hand  with  the 
differentiation  and  integration  of  the  organs.  Between  the 
inquiry  pursued  in  Part  IV,  and  the  inquiry  to  be  pursu:  d 
in  this  Part,  the  contrast  is  that,  in  the  first  place,  facts  of 
structure  are  now  to  be  used  to  interpret  facts  of  function, 
instead  of  conversely;  and,  in  the  second  place,  the  facts 
of  structure  to  be  so  used  are  not  those  of  conspicuous  shape 
so  much  as  those  of  minute  texture  and  chemical  com- 
position. 

§  266.  The  problems  of  Physiology,  in  the  wide  sense 
above  described,  are,  like  the  problems  of  Morphology,  to  be 
considered  as  problems  to  which  answers  must  be  given  in 
terras  of  incident  forces.  On  the  hypothesis  of  Evolution 
these  specializations  of  tissues  and  accompanying  concen- 
trations of  functions,  must,  like  the  specializations  of  shape 
in  an  organism  and  its  component  divisions,  be  due  to  the 
actions  and  reactions  which  its  intercourse  with  the  environ- 
ment involves ;  and  the  task  before  us  is  to  explain  how  they 
tire  wrought — how  they  are  to  be  comprehended  as  results 
of  such  actions  and  reactions. 

Or,  to  define  these  problems  still  more  specifically: — Those 
extremely  unstable  substances  which  compose  the  proto- 
plasm of  which  organisms  are  mainly  built,  have  to  be 
traced  through  the  various  modifications  in  their  properties 
and  powers,  that  are  entailed  on  them  by  changes  of  relation  to 
agencies  of  all  kinds.  Those  organic  colloids  which  pass  from 


221  PHYSIOLOGICAL   DEVELOPMENT. 

liquid  to  solid  and  from  soluble  to  insoluble  on  the  slightest 
molecular  disturbance — those  albumenoid  matters  which,  as 
we  see  in  clotted  blood  or  the  coagulable  lymph  that  is 
poured  out  on  abraded  surfaces  and  causes  adhesion  between 
inflamed  membranes,  assume  new  forms  with  the  greatest 
readiness,  are  to  have  their  metamorphoses  studied  in  con- 
nexion with  the  influences  at  work.  Those  compounds  which, 
as  we  see  in  the  quickly-acquired  brownness  of  a  bitten 
apple  or  in  the  dark  stains  produced  by  the  milky  juice  of  a 
Dandelion,  immediately  begin  to  alter  when  the  surrounding 
actions  alter,  are  to  be  everywhere  considered  as  undergoing 
modifications  by  modified  conditions.  Organic  bodies,  con- 
sisting of  substances  that,  as  I  here  purposely  remind  the 
reader,  are  prone  beyond  all  others  to  change  when  the 
incident  forces  are  changed,  we  must  contemplate  as  in  all 
their  parts  differently  changed  in  response  to  the  different 
changes  of  the  incident  forces.  And  then  we  have  to  re- 
gard the  concomitant  differentiations  of  their  reactions  as 
being  concomitant  differentiations  of  their  functions. 

Here,  as  before,  we  must  take  into  account  two  classes 
of  factors.  We  have  to  bear  in  mind  the  inherited  results  of 
actions  to  which  antecedent  organisms  were  exposed,  and  to 
join  with  these  the  results  of  present  actions.  Each  organism 
is  to  be  considered  as  presenting  a  moving  equilibrium  of 
functions,  and  a  correlative  arrangement  of  structures, 
produced  by  the  aggregate  of  actions  and  reactions  that  have 
taken  place  between  all  ancestral  organisms  and  their  envi- 
ronments. The  tendency  in  each  organism  to  repeat  this 
adjusted  arrangement  of  functions  and  structures,  must  be 
regarded  as  from  time  to  time  interfered  with  by  actions  to 
which  its  inherited  equilibrium  is  not  adjusted — actions  to 
which,  therefore,  its  equilibrium  has  to  be  re-adjusted.  And 
in  studying  physiological  development  we  have  in  all  cases 
to  contemplate  the  progressing  compromise  between  the  old 
and  the  new,  ending  in  a  restored  balance  or  adaptation. 

It  is  manifest  that  our  data  are  so  scanty  that  nothing 


THE    PROBLEMS   OF   PHYSIOLOGY.  225 

more  than  very  general  and  approximate  interpretations  of 
this  kind  are  possible.  If  the  hypothesis  of  Evolution  fur- 
nishes us  with  a  rude  conception  of  the  way  in  which  the 
more  conspicuous  and  important  differentiations  of  functions 
have  arisen,  it  is  as  much  as  can  be  expected. 

§  267.  It  will  be  best,  for  brevity  and  clearness,  to  deal 
with  these  physiological  problems  as  we  dealt  with  the 
morphological  ones — to  carry  on  the  inductive  statement  and 
the  deductive  interpretation  hand-in-hand :  so  disposing 
of  each  general  truth  before  passing  to  the  next.  Treating 
separately  vegetal  organisms  and  animal  organisms,  we  will 
in  each  kingdom  consider: — first,  the  physiological  differentia- 
tions and  accompanying  changes  of  structure  that  arise  be- 
tween outer  tissues  and  inner  tissues  ;  next,  those  that  arise 
between  different  parts  of  the  outer  tissues  ;  and,  finally, 
those  that  arise  between  different  parts  of  the  inner  tissues. 
What  little  has  to  be  said  concerning  physiological  integra- 
tion must  come  last.  For  though,  in  tracing  up  Mor- 
phological Evolution,  we  have  to  study  those  processes  of 
integration  by  which  organic  aggregates  are  formed,  before 
studying  the  differentiations  that  arise  among  their  parts ; 
we  must,  contrariwise,  in  tracing  up  Physiological  Evolution, 
study  the  genesis  of  the  different  functions  before  we  study 
the  inter-dependence  that  eventually  arises  among  them  and 
constitutes  physiological  unity. 


CHAPTER  IT. 

DIFFERENTIATIONS     BETWEEN     THE     OUTER     AND      INNER 
TISSUES    OF    PLANTS 

§  268.  The  simplest  plant  presents  a  contrast  between  its 
peripheral  substance  and  its  central  substance.  In  each  pro- 
topbyte,  be  it  a  spherical  cell  or  a  branched  tube,  or  such 
a  more-specialized  form  as  a  Desmid,  a  marked  unlikeness 
exists  between  the  limiting  layer  and  that  which  it  limits. 
These  vegetal  aggregates  of  the  first  order  may  differ  widely 
from  one  another  in  the  natures  of  their  outer  coats  and  in 
the  natures  of  their  contents.  As  in  a  Palmella,  there  may 
exist  a  clothing  of  jelly  ;  or,  as  in  Diatom,  the  walls  may 
take  the  form  of  silicious  valves  variously  sculptured.  The 
contained  matter  may  be  here  green,  there  red,  and  in  other 
cases  brown  or  black.  But  amid  all  these  diversities  there  ia 
this  one  uniformity  — a  strong  distinction  between  the  parts 
in  contact  with  the  environment  and  the  parts  not  in  con- 
tact with  the  environment. 

When  we  remembsr  that  this  trait  is  one  which  these 
simple  living  bodies  have  in  common  with  bodies  that  are  not, 
living  — when  we  remember  that  each  inorganic  mass  even- 
tually has  its  outer  part  more  or  less  differentiated  from 
its  inner  part,  here  by  oxidation,  there  by  drying,  and  else- 
where by  the  actions  of  light,  of  moisture,  of  frost ;  we  can 
scarcely  resist  the  crnclus!on  that,  in  the  one  case  as  in 


THE    OUTER    AND    INNER   T1SSUFS    OF   PLANTS.  227 

the  other,  the  contrast  is  due  to  the  unlike  actions  to  which 
the  parts  are  subject.  Given  an  originally-homogenous 
portion  of  protoplasm,  and  it  follows  from  the  general  laws 
of  Evolution  (First  Principles,  §3  109 — 115)  first,  that  it  must 
lose  its  homogeneity,  and,  second,  that  the  leading  dissimila- 
rities must  arise  between  the  parts  most-dissimilarly  con- 
ditioned— that  is,  between  the  outside  and  the  inside.  The 
exterior  must  bear  amounts  and  kinds  of  force  unlike  the 
amounts  and  kinds  which  the  interior  bears;  and  from 
the  persistence  of  force  it  follows  inevitably  that  unlike 
effects  must  be  wrought  on  them— they  must  be  differen- 
tiated. 

What  is  the  limit  towards  which  the  differentiation 
tends?  "We  have  seen  that  the  re-distribution  of  matter 
and  motion  whence,  under  certain  conditions,  evolution 
results,  can  never  cease  until  equilibrium  is  reached — proxi- 
raately  a  moving  equilibrium,  and  finally  a  complete  equi- 
librium (First  Principles,  §§  130 — 135).  Hence,  the  diffe- 
rentiation must  go  on  until  it  establishes  such  differences  in 
the  parts  as  shall  balance  the  differences  in  the  forces  acting 
on  them.  When  dealing  with  equilibration  in  general,  we 
saw  that  this  process  is  what  is  called  adaptation  (First 
Principles,  §  133)  ;  and,  more  recently,  we  saw  that  by  it  the 
totality  of  functions  of  an  organism  is  brought  into  cor- 
respondence with  the  totality  of  actions  affecting  it  (§§  159 
— 163).  Manifestly  in  this  case,  as  in  all  others,  either 
death  or  adjustment  must  eventually  result.  A  force  falling 
on  one  of  these  minute  aggregates  of  protoplasm,  must  ex- 
pend itself  in  working  its  equivalent  of  change.  If  this 
force  is  such  that  in  expending  itself  it  disturbs  beyond 
rectification  the  balance  of  the  organic  processes,  then  the 
aggregate  is  disintegrated  or  decomposed.  But  if  it  does 
not  overthrow  that  moving  equilibrium  constituting  the  life 
of  the  aggregate,  then  the  aggregate  continues  in  that  modi- 
fied form  produced  by  the  expenditure  of  the  force.  Thus, 
by  direct  equilibration,  continually  furthered  by  indirect 


228  PHYSIOLOGICAL   DEVELOPMENT. 

equilibration,  there  must  arise  this  distinction  between  the 
outer  part  adapted  to  meet  outer  forces,  and  the  inner  part 
adapted  to  meet  inner  forces.  And  their  respective  actions, 
as  thus  meeting  outer  and  inner  forces,  must  be  what  we  call 
their  respective  functions. 

§  269.  Aggregates  of  the  second  order  exhibit  parallel 
traits,  admitting  of  parallel  interpretations.  Integrated 
masses  of  cells  or  units  homologous  with  protophytes, 
habitually  show  us  contrasts  between  the  characters  of  the 
superficial  tissues  and  the  central  tissues.  Such  among  these 
aggregates  of  the  second  order  as  have  their  component  units 
arranged  into  threads  or  lamina),  single  or  double,  cannot,  of 
course,  furnish  contrasts  of  this  kind ;  for  all  their  units  are 
as  much  external  as  internal.  We  must  turn  to  the  more  or 
less  massive  forms. 

Of  these,  among  Fungi,  the  common  Puff-ball  is  a  good 
example — good  because  it  presents  this  fundamental  differen- 
tiation but  little  complicated  by  others.  In  it  we  have  a 
cortical  layer  of  cellular  tissue  obviously  unlike  the  mass  of 
cellular  tissue  which  it  incloses.  So  far  as  the  unlikeness 
between  external  and  internal  parts  is  concerned,  we  see  here 
a  relation  analogous  to  that  existing  in  the  simple  cell ;  and 
we  see  in  it  a  similar  meaning :  there  is  a  physiological 
differentiation  corresponding  to  the  difference  in  the  incidence 
of  forces. 

Under  various  forms  the  Algce  show  just  the  same  rela- 
tion. Where,  as  in  Codiwn  Bursa,  we  have  ramified  tubuhir 
cells  aggregated  into  a  hollow  globular  mass,  the  outer  and 
inner  surfaces  are  contrasted  both  in  colour  and  structure ; 
though  the  tubules  composing  the  two  surfaces  are  con- 
tinuous with  one  another.  In  Mivularia,  again,  we  see  the 
like,  both  in  the  radial  arrangement  of  the  imbedded  threads 
and  in  the  difference  of  colour  between  the  exterior  of  the 
imbedding  jelly  and  its  interior.  The  more- developed  Alga 
of  all  kinds  repeat  the  antithesis.  In  branched  sterns, 


THE    OUTER    AND    INNER   TISSUES    OF    PLANTS.  229 

when  they  consist  of  more  than  single  rows  of  cells,  the 
outer  cells  become  unlike  the  inner,  as  shown  in  Fig.  35. 
Such  types  as  Chrysymenia  rosea  show  us  this  unlikeness 
very  conspicuously.  And  it  holds  even  with  ribbon-shaped 
fronds.  Wherever  one  of  these  is  composed  of  three,  four, 
or  more  layers,  as  in  Laminariu  and  Pnnctaria,  the  cells  of 
the  external  layers  are  strongly  distinguished  from  those  of 
the  internal  layers,  both  by  their  comparative  smallness  and 
by  their  deep  colour. 

§  270.  The  higher  plants  variously  display  the  like 
fundamental  distinction  between  outer  and  inner  tissues. 
Each  leaf,  thin  as  it  is,  exemplifies  this  differentiation  of  the 
parts  immediately  in  contact  with  the  environment  from  the 
parts  not  in  immediate  contact  with  the  environment.  Its 
cuticular  cells,  forming  a  protecting  envelope,  diverge  physi- 
cally and  chemically  from  the  contained  cells  of  parenchyma, 
which  carry  on  the  more  active  functions.  And  the  contrast 
may  be  observed  to  establish  itself  in  the  course  of  develop- 
ment. At  first  the  component  cells  of  the  leaf  are  all  alike  ; 
and  this  unlikeness  between  the  cells  of  the  outer  and  inner 
layers,  arises  simultaneously  with  the  rise  of  differences  in 
their  conditions — differences  that  have  acted  on  all  ancestral 
leaves  as  they  act  on  the  individual  leaf. 

An  unlikeness  more  marked  in  kind  but  similar  in  mean- 
ing, exists  between  the  bark  of  every  branch  and  the  tissues 
it  clothes.  The  phsenogamic  axis,  especially  when  exogenous, 
is  commonly  characterized  by  an  outer  layer  differing  from 
the  inner  layers  in  character  and  function,  as  it  differs  from 
them  in  position.  Subject  as  this  outer  layer  is  to  the  un- 
mitigated actions  of  forces  around — to  abrasions,  to  extremes 
of  heat  and  cold,  to  evaporation  and  soaking  with  water — its 
units  cannot  cease  changing  until  they  are  in  equilibrium 
with  these  more  violent  actions,  and  have  acquired  molecular 
constitutions  more  stable  than  those  of  the  interior  cells. 
That  is  to  say,  the  forces  which  differentiate  the  cortical  part 


230  PHYSIOLOGICAL   DEVELOPMENT. 

from  the  rest  are  the  forces  which  it  has  to  resist,  and  fron/ 
which  it  passively  protects  the  parts  within.  How 

clearly  this  heterogeneity  of  structure  and  function  is  conse- 
quent upon  intercourse  with  the  environment,  every  tree 
and  shrub  shows.  The  young  shoots,  alike  of  annuals  and 
perennials,  are  quite  green  and  soft  at  their  extremities. 
Among  plants  of  short  lives,  there  is  usually  but  a  slight 
development  of  bark  :  such  traces  of  it  as  the  surface  of  the 
axis  acquires  being  seen  only  at  its  lowermost  or  oldest 
portion.  In  long-lived  plants,  however,  this  formation  of  a 
tough  opaque  coating  takes  place  more  rapidly ;  and  shows 
us  distinctly  the  connexion  between  the  degree  of  differentia- 
tion and  the  length  of  exposure.  For,  in  a  growing  twig, 
we  see  that  the  bark,  invisible  at  the  bud,  thickens  by 
insensible  gradations  as  we  go  downwards  to  the  junction  of 
the  twig  with  the  branch  ;  and  we  come  to  still  thicker  parts 
of  it  as  we  descend  along  the  branch  towards  the  main  stem. 
Moreover,  on  examining  main  stems  we  find  that  while  in 
some  trees  the  bark,  cracked  by  expansion  of  the  wood,  drops 
off  in  flakes,  leaving  exposed  patches  of  the  inner  tissue  which 
presently  become  green  and  finally  develop  new  cortical 
layers ;  in  other  trees  the  exfoliated  flakes  continue  adherent, 
and  in  the  course  of  years  form  a  rugged  fissured  coat :  so 
producing  a  still  more  marked  contrast  between  outside  and 
inside.  Of  course  the  establishment  of  this  he- 

terogeneity is  furthered  by  natural  selection,  which,  where  a 
protective  covering  is  needed,  gives  an  advantage  to  those 
individuals  in  which  it  has  become  strongest.  But  that  this 
divergence  of  structure  commences  as  a -direct  adaptation,  is 
clearly  shown  by  other  facts  than  the  foregoing.  There  is 
the  fact  that  many  of  the  plants  which  in  our  gardens 
develop  bark  with  considerable  rapidity,  do  not  develop  it 
with  the  same  rapidity  in  a  greenhouse.  And  there  is  the 
fact  that  plants  which,  in  some  climates,  have  their  stems 
covered  only  by  thin  semi-transparent  layers,  acquire  thick 
opaque  layers  when  taken  to  other  climates. 


THE    OUTER   ASD    IXXER   TISSUES    OF    PLANTS.  231 

Just  noting,  for  the  sake  of  completeness,  that  in  the 
roots  of  the  higher  plants  there  arises  a  contrast  between 
outer  and  inner  parts,  parallel  to  the  one  we  have  traced  in 
their  branches,  let  me  draw  attention  to  another  differentia- 
tion of  the  same  ultimate  nature,  which  the  higher  plants 
exhibit  to  us — a  differentiation  which,  familiar  though  it  is, 
gains  a  new  meaning  by  association  with  those  named  above, 
and  makes  their  meaning  still  more  manifest.  I  refer  to  the 
fact  that  when,  by  the  budding  of  axes  out  of  axes,  there 
is  produced  one  of  those  highly-compounded  Phocnogams 
which  we  call  a  tree,  the  central  part  of  the  aggregate  be- 
comes functionally  and  structurally  unlike  the  peripheral 
part.  On  looking  into  a  large  tree,  or  even  a  small  one 
that  has  thick  foliage,  like  the  Laurel,  we  see  that  the  in- 
ternal branches  are  almost  or  quite  bare  of  leaves,  while  the 
leaf-clad  branches  form  an  external  stratum  ;  and  all  our 
experience  unites  in  proving  that  this  contrast  arises  by 
degrees,  as  fast  as  the  growth  of  the  tree  entails  a  contrast  be- 
tween the  conditions  to  which  inner  and  outer  branches  are 
exposed.  Now  when,  in  these  most- composite  aggregates, 
we  see  a  differentiation  between  peripheral  and  central  parts 
clemonstrably  caused  by  a  difference  in  the  relations  of  these 
parts  to  environing  forces,  we  get  support  for  the  conclusion 
otherwise  reached,  that  there  is  a  parallel  cause  for  the  parallel 
differentiations  exhibited  by  all  aggregates  of  lower  orders — 
branches,  leaves,  cells. 

§  271.  Before  leaving  this  most  general  physiological 
differentiation,  it  may  be  well  to  say  something  respecting 
certain  secondary  unlikenesses  that  habitually  arise  be- 
tween interior  and  exterior.  For  the  contrast  is  not,  as 
might  be  supposed  from  the  foregoing  descriptions,  a  simple 
contrast:  it  is  a  compound  contrast.  The  outer  structure 
itself  is  usually  divisible  into  concentric  structures.  This 
is  equally  true  of  a  protophyte  and  of  a  phaonogamic  axis. 
Between  the  centre  of  an  independent  vegetal  cell  and  ita 


232  PHYSIOLOGICAL   DEVELOPMENT. 

surface,  there  are  at  least  two  layers  ;  and  the  bark  coating 
the  substance  of  a  shoot,  besides  being  itself  compound, 
includes  another  tissue  lying  between  it  and  the  wood.  "What 
is  the  physical  interpretation  of  these  facts  ? 

When  a  mass  of  what  we  distinguish  as  inert  matter  is 
exposed  to  external  agencies  capable  of  working  changes  in 
it — when  it  is  chemically  acted  upon,  or  when,  being  dry,  it 
is  allowed  to  soak,  or  when,  being  wet,  it  is  allowed  to  dry — 
the  changes  set  up  progress  in  an  equable  way  from  the 
surface  towards  the  centre.  At  any  time  during  the  process 
(supposing  no  other  action  supervenes)  the  modification 
wrought,  first  completed  at  the  outside,  either  gradually 
diminishes  as  we  approach  the  centre,  or  ceases  suddenly 
at  a  certain  distance  from  the  centre.  But  now  suppose  that 
the  mass,  instead  of  being  inert,  is  the  seat  of  active  changes 
• — suppose  that  it  is  a  portion  of  complex  colloidal  substance, 
permeable  by  light  and  by  fluids  capable  of  affecting  its 
unstable  molecules — suppose  that  its  interior  is  a  source  of 
forces  continually  liberated  and  diffusing  themselves  out- 
wards. Is  it  not  likely  that  while  at  the  centre  the  action 
of  the  internally-liberated  forces  will  dominate,  and  while  at 
the  surface  the  action  of  the  environing  forces  will  dominate, 
there  will  be  between  the  two  a  certain  place  at  which  their 
actions  balance  ?  And  may  we  not  expect  that  this  will  be 
the  place  where  the  most  unstable  matter  exists — the  place 
outside  of  which  the  matter  becomes  relatively  stable  in  the 
face  of  external  forces,  and  inside  of  which  the  matter  be- 
comes relatively  stable  in  the  face  of  internal  forces  ? 

Be  this  or  be  this  not  the  explanation,  the  well-known  fact 
is  that  the  inner  wall  of  each  vegetal  cell  is  a  delicate  mem- 
brane, the  primordial  utricle,  composed  of  that  nitrogenous 
substance  specially  characterized  by  instability;  and  that 
outside  of  this  is  the  cellulose  layer,  and  inside  of  it  the 
granular  colouring  matter.  And  the  similarly  well-known 
fact  is,  that  in  each  phscnogamic  axis  the  cambium  layer, 
which  shows  its  relative  instability  by  the  activity  of  the 


THE    OUTER    AND    INNER   TISSUES    OF    PLANTS.  233 

changes  going  on  in  it,  lies  between  the  bark  and  the  mass 
of  the  axis ;  and  is  the  place  from  which  the  differentiations 
producing  these  proceed  in  opposite  directions. 

But  we  are  here  chiefly  concerned  with  the  more  general 
interpretation,  which  is  independent  of  any  such  speculation 
as  the  foregoing.  These  contrasted  tissues  and  the  contrasted 
functions  they  severally  perform  are,  beyond  question,  sub- 
ordinated to  the  relations  of  outside  and  inside.  And  the 
evidence  makes  it  tolerably  clear  that  the  unlike  actions  of 
forces  involved  by  the  relations  of  outside  and  inside,  deter- 
mine these  contrasts — partly  directly  and  partly  indirectly. 


CHAPTER  HI. 


DIFFERENTIATIONS  AMONG  THE  OUTER  TISSUES  OF  PLANTS. 

§  272.  The  Protococci  and  such  compound  forms  as  tbe 
Volvox  globator,  which  do  not  permanently  expose  any  parts 
of  their  surfaces  to  actions  unlike  those  which  other  parts 
are  exposed  to,  have  no  parts  of  their  surfaces  unlike  the 
rest  in  function  and  composition.  This  is  what  the  hypo- 
thesis prepares  us  for.  If  physiological  differentiations  are 
determined  by  differences  in  the  incidence  of  forces,  then 
there  will  be  no  such  differentiations  where  there  are  no 
such  differences.  Contrariwise,  it  is  to  be  expected  that  the 
most  conspicuous  unlikeness  of  function  and  minute  structure 
will  arise  between  the  most-dissimilarly  circumstanced  parts 
of  the  surface.  We  find  that  they  do.  The  upper  end  and  the 
lower  end,  or,  more  strictly  speaking,  the  free  end  and  the 
attached  end,  habitually  present  the  strongest  physiological 
contrasts. 

Even  aggregates  of  the  first  order  illustrate  this  truth. 
Such  so-called  unicellular  plants  as  those  delineated  in 
Figs  4,  5,  and  6,  show  us,  on  comparing  the  contents  of 
their  fixed  ends  and  their  loose  ends,  that  different  processes 
are  going  on  in  them,  and  that  different  functions  are 
being  performed  by  their  limiting  membranes.  Caulerpa 
prolifera,  which  '-'  consists  of  a  little  creeping  stem  w.'th 
roots  below  and  leaves  above,"  originating  "  in  the 


THE    OUTER   TISSUES    OF   PLANTS.  235 

growth  of  a  body  which  may  be  regarded  as  an  individual 
cell,"  supplies  a  still-better  example.  Among 

aggregates  of  the  second  order  the  like  connexion  is 
displayed  in  more  various  modes  but  with  equal  con- 
sistency. As,  before,  the  Puff-ball  served  to  exemplify  the 
primary  physiological  differentiation  of  outer  parts  from 
inner  parts ;  so,  here,  it  supplies  a  simple  illustration  of 
the  way  in  which  the  differentiated  outer  part  is  re-dif- 
ferentiated, in  correspondence  with  the  chief  contrast  in  its 
relations  to  the  environment.  The  only  marked  unlikeness 
which  the  cortical  layer  of  the  Puff-ball  presents,  is  that 
between  the  portion  next  the  ground  and  the  opposite  portion. 
The  better-developed  Fungi  exhibit  a  more  decided  hetero- 
geneity of  parallel  kind.  Such  incrusting  Alyce  as  Rulfsia 
deusta  furnish  a  kindred  contrast ;  and  in  the  higher  Algaz 
it  .is  uniformly  repeated.  Phaenogams  display 

this  physiological  differentiation  very  conspicuously.  That 
earth  and  air  are  unlike  portions  of  the  environment,  sub- 
jecting roots  and  leaves  to  unlike  physical  forces,  which  entail 
on  them  unlike  reactions  ;  and  that  the  unlike  functions  and 
structures  of  their  respective  surfaces  are  fitted  to  these 
unlike  physical  forces ;  are  familiar  facts  which  it  would  be 
needless  here  to  name,  were  it  not  that  they  must  be  counted 
as  coming  within  a  wider  group  of  facts. 

Is  this  unlikeness  between  the  outer  tissues  of  the  attached 
ends  and  those  of  the  free  ends  in  plants,  determined  by 
their  converse  with  the  unlike  parts  of  the  environment  ? 
That  they  result  from  an  equilibration  partly  arising  in 
the  individual  and  partly  arising  by  the  survival  of  indivi- 
duals in  which  it  has  been  carried  furthest,  is  inferable 
a  priori;  and  this  d  priori  argument  may  be  adequately 
enforced  by  arguments  of  the  inductive  order.  A  few 
typical  ones  must  here  suffice.  The  gemmulea 

of  the  Marchantia  are  little  disc-shaped  masses  of  cells 
composed  of  two  or  more  layers.  Their  sides  being  alike, 
there  is  nothing  to  determine  which  side  falls  lowermost 


236  PHYSIOLOGICAL   DEVELOPMENT. 

when  one  of  them  is  detached.  "Whichever  side  falls  lower- 
most, however,  presently  begins  to  send  out  rootlets,  while  the 
uppermost  side  begins  to  assume  those  characters  which 
distinguish  the  face  of  the  frond.  When  this  differentiation 
has  commenced,  the  tendency  to  its  complete  establishment 
becomes  more  and  more  decided ;  as  is  proved  by  the  fact 
that  if  the  positions  of  the  surfaces  be  altered,  the  gemrnule 
bends  itself  so  as  to  re-adjust  them :  the  change  towards 
equilibrium  with  environing  forces  having  been  once  set  up, 
there  is  acquired,  as  it  were,  an  increasing  momentum  which 
resists  any  counter-change.  But  the  evidence  shows  that 
at  the  outset,  the  relations  to  earth  and  air  alone  deter- 
mine the  differentiation  of  the  under  surface  from  the 
upper.  The  experiences  of  the  gardener,  multi- 

plying his  plants  by  cuttings  and  layers,  constitute  another 
class  of  evidences  not  to  be  omitted  :  they  are  commonplace 
but  instructive  examples  of  physiological  differentiation. 
While  circumstanced  as  it  usually  is,  the  cambium  of  each 
branch  in  a  Phaenogam  continues  to  perform  its  ordinary 
function — transforming  itself  on  its  outer  side  into  the 
cortical  substances,  and  on  its  inner  side  into  vascular  and 
woody  tissues.  But  change  the  conditions  to  those  which 
the  underground  part  of  the  plant  is  exposed  to,  and  there 
begins  another  differentiation  resulting  in  underground  struc- 
tures. Contact  with  water  often  suffices  alone  to  produce 
this  result,  as  in  the  branches  of  some  trees  when  they  droop 
into  a  pool,  or  as  occasionally  with  a  cutting  placed  in  a 
bottle  of  water ;  and  when  the  light  is  excluded  by  im- 
bedding the  end  of  the  cutting,  or  the  middle  of  the  still- 
attached  branch,  in  the  earth,  this  production  of  tissues 
adapted  to  the  function  of  absorbing  moisture  and  mineral 
constituents  proceeds  still  more  readily.  With  such  cases 
may  be  grouped  those  in  which  this  development  of  under- 
ground organs  by  an  above-ground  tissue,  is  not  excep- 
tional but  habitual.  Creeping  plants  furnish  good  illus- 
trations. From  the  shoots  of  the  Ground- Ivy,  rootlets  an* 


THE    OUTER   TISSUES   OF    PLANTS.  237 

put  out  into  the  soil  in  a  manner  differing  but  little  from 
that  in  which  they  are  put  out  by  an  imbedded  layer ;  save 
that  the  process  follows  naturally-induced  conditions  instead 
of  following  artificially-induced  conditions.  But  in  the 
common  Ivy  which,  instead  of  running  along  the  surface 
of  the  earth,  runs  up  inclined  or  vertical  surfaces,  we  see  the 
process  interestingly  modified  without  being  essentially 
changed.  The  rootlets,  here  differentiated  by  their  con- 
ditions into  organs  of  attachment  much  more  than  organs  of 
absorption,  still  develop  on  that  side  of  the  shoot  next  the 
supporting  surface,  and  do  not  develop  where  the  shoot, 
growing  away  from  the  tree  or  wall,  is  surrounded  equally 
on  all  sides  by  light  and  air — thus  showing,  undeniably, 
that  the  production  of  the  rootlets  is  determined  by  the 
differential  incidence  of  forces.  That  greenness 

which  may  be  observed  in  these  Ivy-branch  rootlets  while 
they  are  quite  young,  soft,  and  unshaded,  introduces  us  to 
facts  which  are  the  converse  of  the  foregoing  facts  ;  and  prove 
that  the  parts  ordinarily  imbedded  in  the  soil  and  adapted  to 
its  actions,  acquire,  often  in  a  very  marked  degree,  the  super- 
ficial structures  of  the  aerial  parts,  when  they  are  exposed  to 
light  and  air.  This  may  be  witnessed  in  Maize,  which,  when 
luxuriant,  sends  out  from  its  nodes  near  the  ground,  clusters 
of  roots  that  are  thick,  succulent,  and  of  the  same  colour  as 
the  leaves.  Examples  more  familiar  to  us  in  England,  occur 
in  every  field  of  Turnips.  On  noting  how  green  is  the  un- 
covered part  of  a  Turnip-root,  and  how  manifestly  the 
area  over  which  the  greenness  extends  varies  with  the  area 
exposed  to  light,  as  well  as  with  the  degree  of  the  exposure, 
it  will  be  seen  that  beyond  question,  root-tissue  assumes 
to  a  considerable  extent  the  appearance  and  function  of 
leaf-tissue,  when  subject  to  the  same  agencies.  Let  us  not 
forget,  too,  that  where  exposed  roots  do  not  approach  in 
superficial  character  towards  leaves,  they  approach  in 
superficial  character  towards  stems — becoming  clothed 
with  a  thick,  fissured  bark,  like  that  of  the  trunk  and 


238  PHYSIOLOGICAL   DEVELOPMENT. 

branches.  But  the  most  conclusive  evidence  is 

furnished  by  the  actual  substitutions  of  surface- structures 
and  functions,  that  occur  in  aerial  organs  which  have  taken 
to  growing  permanently  under  ground,  and  in  under-ground 
organs  which  have  taken  to  growing  permanently  in  the 
air.  On  the  one  hand,  there  is  the  rhizoma,  exemplified  by 
Ginger — a  stem  which,  instead  of  shooting  up  vertically, 
runs  horizontally  below  the  surface  of  the  soil,  and  assumes 
the  character  of  a  root,  alike  in  colour,  texture,  and 
production  of  rootlets;  and  there  is  that  kind  of  swollen 
under-ground  axis,  bearing  axillary  buds,  which  the  Potato 
exemplifies — a  structure  which,  though  homologically  an 
axis,  simulates  a  tuberous  root  in  surface-character,  and 
when  exposed  to  the  air,  manifests  no  greater  readiness 
to  develop  chlorophyll  than  a  tuberous  root  does.  On  the 
other  hand,  there  are  the  aerial  roots  of  certain  Orchids, 
which,  habitually  green  at  their  tips,  continue  green 
throughout  their  whole  lengths  when  kept  moist ;  which 
have  become  leaf-like  not  only  by  this  development  of 
chlorophyll,  but  also  by  the  acquirement  of  stomata ;  and 
which  do  not  bury  themselves  in  the  soil  when  they  have 
the  opportunity.  Thus  we  have  aerial  organs  so  com- 
pletely changed  to  fit  under-ground  actions,  that  they  will 
not  resume  aerial  functions ;  and  under-ground  organs  so 
completely  changed  to  fit  aerial  actions,  that  they  will  not 
resume  under- ground  functions. 

That  the  physiological  differentiation  between  the  part  of 
a  plant's  surface  which  is  exposed  to  light  and  air  and  the 
part  which  is  exposed  to  darkness  and  moisture  and  solid 
matter,  is  primarily  due  to  the  unlike  actions  of  these 
unlike  parts  of  the  environment,  is,  then,  clearly  implied  bv 
observed  facts — more  clearly,  indeed,  than  was  to  be  expected. 
Considering  how  strong  must  be  the  inherited  tendency 
of  a  plant  to  assume  those  special  characters,  physio- 
logical as  well  as  morphological,  which  have  resulted  from 
an  enormous  accumulation  of  antecedent  actions,  it  may 


TEIE    OUTER   TISSUES   OF    PLANTS.  239 

be  even  thought  surprising  that  this  tendency  can 
be  counteracted  to  so  great  an  extent  by  changed  con- 
ditions. Such  a  degree  of  modifiability  becomes  compre- 
hensible, only  when  we  remember  how  little  a  plant's 
functions  are  integrated ;  and  how  much,  therefore,  the 
functions  going  on  in  each  part  may  be  altered  without 
having  to  overcome  the  momentum  of  the  functions  through 
out  the  whole  plant.  But  this  modifiability  being  as  great 
as  it  is,  we  can  have  no  difficulty  in  understanding  how,  by 
the  cumulative  aid  of  natural  selection,  this  primary  differen- 
tiation of  the  surface  in  plants  has  become  what  we  see  it. 

§  273.  We  will  leave  now  these  contrasts  between  the 
free  surfaces  of  plants  and  their  attached  or  imbedded 
surfaces,  and  turn  our  attention  to  the  secondary  contrasts 
existing  between  different  parts  of  their  free  surfaces.  Were 
a  full  statement  of  the  evidence  practicable,  it  would  be 
proper  here  to  dwell  on  that  which  is  furnished  by  the 
inferior  classes.  It  might  be  pointed  out  in  detail  that 
where,  as  among  the  Algce,  the  free  surfaces  are  not  dis- 
similarly conditioned,  there  is  no  systematic  differentiation  of 
them — that  the  frond  of  an  Viva,  the  ribbon-shaped  divisions 
of  a  Laminaria,  and  the  dichotomous  expansions  of  the  Fuci 
that  clothe  the  rocks  between  tide-marks,  are  alike  on  both 
Bides ;  because,  swayed  about  in  all  directions  as  they  are  by 
the  waves  and  tides,  their  sides  are  equally  affected.  Con- 
versely, from  the  Fungi  might  be  drawn  abundant  proof  that 
even  among  Thallogens,  unlikenesses  arise  between  different 
parts  of  the  free  surfaces  when  their  circumstances  are  unlike 
— that  in  such  laterally-growing  kinds  as  are  shown  in  Fig. 
196  b,  the  honeycombed  under  surface  and  the  smooth 
leathery  upper  surface,  have  their  contrasts  related  to  con- 
trasted conditions ;  and  that  in  the  adjacently- figured 
Agarics,  and  other  stalked  genera,  the  pileus  exhibits  a 
parallel  difference,  explicable  in  a  parallel  way.  But  passing 
over  Cryptogams,  it  must  suffice  if  we  examine  more  at 
VQI..  IJ,  11 


.240  PHYSIOLOGICAL   DEVELOPMENT. 

length  these  traits  as  they  are  displayed  by  Phaenogams.  Let 
us  first  note  the  dissimilarities  between  the  outer  tissues  of 
stems  and  leaves. 

That  these  dissimilarities  arose  by  degrees,  as  fast  as  the 
units  of  which  the  phoenogamic  axis  is  composed  became 
integrated,  is  a  conclusion  in  harmony  with  the  truth  that  in 
every  shoot  of  every  plant,  they  are  at  first  slight  and  become 
gradually  marked.  Already,  in  briefly  tracing  the  contrasts 
between  the  outer  and  inner  tissues  of  plants,  some  facts 
have  been  named  showing,  by  implication,  how  the  cessa- 
tion of  the  leaf-function  in  axes  is  due  to  that  change  of 
conditions  entailed  by  the  discharge  of  other  functions.  Here 
we  have  to  consider  more  closely  facts  of  this  class,  together 
with  others  immediately  to  the  point.  On  pulling 

off  from  a  stem  of  grass  the  successive  sheaths  of  its  leaves, 
the  more-inclosed  parts  of  which  are  of  a  fainter  green  than 
the  outer  parts,  it  will  be  found  that  the  tubular  axis  even- 
tually reached  is  of  a  still  fainter  green  ;  but  when  the  axis 
eventually  shoots  up  into  a  flowering  stem,  its  exposed  part 
acquires  as  bright  a  green  as  the  leaves.  In  other  Eudogens, 
the  leaf-sheaths  of  which  are  successively  burst  and  exfo- 
liated by  the  swelling  axis,  it  may  be  observed  that  where 
the  dead  sheaths  do  not  much  obstruct  the  light  and  air, 
the  surface  of  the  axis  underneath  is  full  of  chlorophyll. 
Dendrobium  is  an  example.  But  when  the  dead  sheaths 
accumulate  into  an  opaque  envelope,  the  chlorophyll  dis- 
appears, and  also,  we  may  infer,  the  function  its  presence 
habitually  implies.  Carrying  with  us  this  evidence,  we  shall 

,.  recognize  a  like  relation  in  Exogens.  While  its  outer  layer 
remains  tolerably  transparent,  an  exogenous  stem  or  branch 
continues  to  show,  by  the  formation  of  chlorophyll,  that  it 
shares  in  the  duties  of  the  leaves ;  but  in  proportion  as  a 
bark  which  the  light  cannot  penetrate  is  produced  by  the 
adherent  flakes  of  dead  skin,  or  by  the  actual  deposit  of  a 
protective  substance,  the  differentiation  of  duties  becomes 
more  decided.  Cactuses  and  Euphorbias  supply 


THE   OUTER   TISSUES   OF   PLAKTS.  241 

us  with  converse  facts  having  the  same  implication.  The 
swollen  succulent  axes  so  strangely  combined  in  these  plants, 
maintain  for  a  long  time  the  transparency  of  their  outer 
layers  ;  and  doing  this,  they  so  efficiently  perform  the  offices 
of  leaves  that  leaves  are  not  produced.  In  some  cases,  axes 
that  are  not  succulent  participate  largely  in  the  leaf-func- 
tion, or  entirely  usurp  it — still,  however,  by  fulfilling  the 
same  essential  conditions.  Occasionally,  as  in  Statice  bras- 
sicceplia,  stems  become  fringed  ;  and  the  fringes  they  bear 
assume,  along  with  the  thinness  of  leaves,  their  darker  green 
and  general  aspect.  In  the  genus  Ruscus,  the  flattened  axis 
simulates  so  closely  the  leaf-structure,  that  were  it  not  for  tho 
flower  borne  on  its  midrib,  or  edge,  its  axial  nature  would 
hardly  be  suspected.  And  let  us  not  omit  to  note  that  where 
axes  usurp  the  characters  of  leaves,  in  their  attitudes  as  well 
as  in  their  shapes  and  thickness,  there  exist  contrasts  between 
their  under  and  upper  surfaces,  answering  to  the  contrasts 
between  the  relations  of  these  surfaces  to  the  light.  Of  this 
Ituscus  androyynus  furnishes  a  striking  example.  In  it  the 
difference  which  the  unaided  eye  perceives  is  much  less  con- 
spicuous than  that  disclosed  by  the  microscope  ;  for  I  find 
that  while  the  face  of  the  pseudo-leaf  has  no  stomata,  the  back 
is  abundantly  supplied  with  them.  One  more  illustration  must 
be  added.  Equally  for  the  morphological  and  physiological 
truths  which  it  enforces,  the  Coccoloba  platycladon  is  one  of 
the  most  instructive  of  plants.  In  it  the  simulation  of  forms 
and  usurpation  of  functions,  are  carried  out  in  a  much  more 
marvellous  way  than  among  the  Cactacetc.  Imagine  a  growth 
resembling  in  outline  a  very  long  willow-leaf,  but  without  a 
midrib,  and  having  its  two  surfaces  alike.  Imagine  that 
across  this  thin,  green,  semi-transparent  structure,  there  are 
from  ten  to  thirty  divisions,  which  prove  to  be  the  successive 
nodes  of  an  axis.  Imagine  that  along  the  edges  of  this 
leaf-shaped  aggregate  of  internodes,  there  arise  axillary 
.  buds,  some  of  which  unfold  into  flowers,  and  others  of  which 
shoot  up  vertically  into  growths  like  the  one  which  bears 


242  PHYSIOLOGICAL    DEVELOPMENT. 

them.  Imagine  a  whole  plant  thus  seemingly  composed  of 
jointed  willow-leaves  growing  from  one  another's  edges, 
and  some  conception  will  be  formed  of  the  Coccoloba.  The 
two  facts  which  have  meaning  for  us  here  are — first,  that  the 
performance  of  leaf-functions  by  these  axes  goes  along  with 
the  assumption  of  a  leaf-like  translucency  ;  and,  second, 
that  these  flattened  axes,  retaining  their  upright  attitudes, 
and  therefore  keeping  their  two  faces  similarly  conditioned, 
have  these  two  faces  alike  in  colour  and  texture. 

That  physiological  differentiation  of  the  surface  which 
arises  iu  Pheenogams  between  axial  organs  and  foliar  organs, 
is  thus  traceable  with  tolerable  clearness  to  those  differences 
between  their  conditions  which  integration  has  entailed — 
partly  in  the  way  above  described,  and  partly  in  other  ways 
still  to  be  named.  By  its  relative  position,  as  being  shaded 
by  the  leaves,  the  axis  is  less- favourably  circumstanced  for 
performing  those  assimilative  actions  effected  by  the  aid  of 
light.  Further,  that  relatively-small  ratio  of  surface  to 
mass  in  the  axis,  which  is  necessitated  by  its  functions  as  a 
support  and  a  channel  for  circulation,  prevents  it  from  taking 
in,  with  the  same  facility  as  the  leaves,  those  surrounding 
gases  from  which  matter  is  to  be  assimilated.  Both  these 
special  causes,  however,  in  common  with  that  previously 
assigned,  fall  within  the  general  cause.  And  in  the  fact  that 
where  the  differential  conditions  do  not  exist,  the  physio- 
logical differentiation  does  not  arise,  or  is  obliterated,  we 
have  clear  proof  that  it  is  determined  by  unlikenesses  in  the 
relations  of  the  parts  to  the  environment. 

§  274.  From  this  most  general  contrast  between  aerial 
surface-tissues — those  of  axes  and  those  of  fblia — we  pass  now 
to  the  more  special  contrasts  of  like  kind  existing  in  folia 
themselves.  Leaves  present  us  with  superficial  differentia- 
tions of  structure  and  function  ;  and  we  have  to  consider  tho 
relations  between  these  and  the  environing  forces. 

Over  the  whole  surface  of  every  phaenogamic  leaf,  as  over 


THE    OUTER   TISSUES    OF    PLANTS.  243 

tho  frcnds  of  the  higher  Acrogens,  there  extends  a  simple  or 
compound  cuticular  layer,  formed  of  cells  that  are  closely 
united  at  their  edges  and  devoid  of  that  granular  colouring 
matter  contained  in  the  layers  of  cells  .they  inclose :  the 
result  being  that  the  membrane  formed  of  them  is  compara- 
tively transparent.  On  the  submerged  leaves  of  aquatic 
Phaenogams,  this  outer  layer  is  thin,  delicate,  and  permeable 
by  water ;  but  on  leaves  exposed  to  the  air,  and  especially 
on  their  upper  surfaces,  it  is  comparatively  strong,  dense, 
often  smooth,  and  impermeable  by  water :  being  thus  fitted 
to  prevent  the  rapid  escape  of  the  contained  juices  by  evapo- 
ration. Similarly,  while  the  leaves  of  terrestrial  plants  that  live 
in  temperate  climates,  usually  have  comparatively  thin  coats 
thus  composed,  in  climates  that  are  both  hot  and  dry,  leaves 
are  commonly  clothed  with  two,  three,  or  more  layers  of  such 
cells.  Nor  is  this  all.  The  outside  of  an  aerial  leaf  differs  from, 
that  of  a  submerged  leaf  by  containing  a  deposit  of  waxy  sub- 
stance. Whether  this  be  exuded  by  the  exposed  surfaces 
of  the  cells,  as  some  contend,  or  whether  it  is  deposited  within 
the  cells,  as  thought  by  others,  matters  not  in  so  far  as  tho 
general  result  is  concerned.  In  either  case  a  waterproof 
coating  is  formed  at  the  outermost  sides  of  these  outermost 
cells  ;  and  in  many  cases  produces  that  polish  by  which  the 
upper  surface  of  the  leaf  is  more  or  less  distinguished  from 
the  under  surface.  This  external  pellicle  pre- 

sents us  with  another  contrast  of  allied  meaning.  On  tho 
upper  surfaces  of  leaves  subject  to  the  direct  action  of  the 
sun's  rays,  there  are  either  few  or  none  of  those  minuto 
openings,  or  stomata,  through  which  gases  can  enter  or 
escape ;  but  on  the  under  surfaces  these  stomata  are  abun  • 
dant — a  distribution  which,  while  permitting  free  absorp- 
tion of  the  needful  carbonic  acid,  puts  a  check  on  tho 
exit  of  watery  vapour.  Two  general  exceptions  to  this  ar- 
rangement may  be  noted.  Leaves  that  float  on  the  water 
have  all  their  stomata  on  their  upper  sides,  and  leaves  that  aro 
submerged  have  no  stomata — modifications  obviously  ap- 


244  PHYSIOLOGICAL    DEVELOPMENT. 

propriatc  to  the  conditions.  What  is  to  be 

said  respecting  the  genesis  of  these  differentiations?  For 
the  last  there  seems  no  direct  cause  :  its  cause  must  be  in- 
direct. The  unlike  actions  to  which  the  upper  and  under 
surfaces  of  leaves  are  subject,  have  no  apparent  tendency  to 
produce  unlikeness  in  the  number  of  their  breathing  holes. 
Here  the  natural  selection  of  spontaneous  variations  furnishes 
the  only  feasible  explanation.  For  the  first,  however,  there 
is  a  possible  cause  in  the  immediate  actions  of  incident 
forces,  which  survival  of  the  fittest  continually  furthers.  The 
substance  contained  in  the  cells  of  leaves  consists  partly 
of  wax  and  partly  of  chlorophyll.  According  to  Mulder, 
"  there  is  a  genetic  connexion  between  the  production  of  wax 
and  that  of  the  green  colouring  matter  in  the  leaves  ;"  and 
he  alleges,  as  the  result  of  his  own  experiments  and  those  of 
Berzelius,  that  chlorophyll  "  may  be  decomposed,  both  by 
oxidizing  and  de-oxidizing  substances,  so  as  to  become  colour- 
less at  last ;  and  that  wax  seems  to  be  producible  from  it  by 
de-oxidizing  actions."  Now  the  superficial  cells  of  leaves 
are  more  exposed  to  the  de-oxidizing  influence  of  light  than 
the  inner  cells ;  those  forming  the  upper  surface  are  more 
exposed  to  it  than  those  forming  the  under  surface ;  and 
those  which  coat  leaves  in  hot  dry  climates  are  more  exposed 
to  it  than  those  by  which  leaves  in  temperate  climates  arc 
coated.  May  it  not  be  that  the  action  of  light,  whence 
chlorophyll  results  as  a  transitional  compound  which  after- 
wards passes  into  a  colourless  compound,  is  an  action  directly 
tending  to  form  these  bleached  and  transparent  outer  layers ; 
and  directly  tending  to  produce  a  greater  thickness  of  such 
layers  in  proportion  as  it  is  intense  ?  There  are  difficulties 
in  the  way  of  this  supposition  ;  for  I  learn  from  Dr.  Hooker 
that  some  of  the  Balanophora,  which  grow  in  the  shade,  are 
very  full  of  wax.  As  these  are  parasites,  however,  and  absorb 
the  prepared  juices  of  other  plants,  the  comparison  is  interfered 
with.  But  whatever  be  its  origin,  we  have  to  note  that  this 
waxy  substance  suspended  in  the  fluid  which  these  bleached 


THE    OUTEU   TISSUES    OF    PLANTS.  245 

outer  cells  contain,  must  be  deposited  as  fast  as  the  fluid 
escapes.  Where  will  it  be  deposited  ?  The  fluid  exhaling 
through  the  walls  of  the  cells  next  the  air,  will  be  likely  to 
leave  behind  the  suspended  substance  attached  to  these  walls. 
On  remembering  the  pellicle  that  is  apt  to  form  on  thick 
solutions  or  emulsions  as  they  dry,  and  how  this  pellicle  as  it 
grows  retards  the  further  drying,  it  will  be  perceived  that 
the  deposit  of  waxy  substance  next  to  the  outer  surfaces  of 
the  cuticular  cells  in  leaves,  is  probably  initiated  by  the 
evaporation  which  it  eventually  checks.  We  have  here, 
indeed,  a  very  simple  case  of  equilibration.  Where  the  loss 
of  water  is  too  great,  this  waxy  pellicle  left  behind  by  the 
escaping  water  will  protect  most  those  individuals  of  the 
species  in  which  it  is  thickest  or  densest ;  and  by  inheritence 
and  continual  survival  of  the  fittest,  there  will  be  established 
in  the  species  that  thickness  of  the  layer  which  brings  the 
evaporation  to  a  balance  with  the  supply  of  water. 

Another  superficial  differentiation,  still  more  familiar,  has 
to  be  noted.  Every  child  soon  learns  to  distinguish  by  its 
colour  the  upper  side  of  a  leaf  from  its  under  side,  if  the  leaf 
is  one  that  has  grown  in  such  way  as  to  establish  the  rela- 
tions of  upper  and  under.  The  upper  surfaces  of  leaves  are 
habitually  of  a  deeper  green  than  the  under.  Microscopic 
examination  shows  that  this  deeper  green  results  from  the 
closer  clustering  of  those  parenchyma-cells  full  of  chlorophyll 
that  are  in  some  way  concerned  with  the  assimilative  actions; 
while  beneath  them  are  more  numerous  intercellular  passages 
communicating  with  those  openings  or  stomata  through  which 
is  absorbed  the  needful  air.  Now  when  it  is  remembered  that 
the  formation  of  chlorophyll  is  clearly  traceable  to  the  action 
of  light — when  it  is  remembered  that  leaves  are  pale  where 
they  are  much  shaded  and  colourless  when  developed  in  the 
dark,  as  in  the  heart  of  a  Cabbage — when  it  is  remembered 
that  succulent  axes  and  petioles,  like  those  of  Sea-kale  and 
Celery,  remain  white  while  the  light  is  kept  from  them  and 
become  green  when  exposed ;  it  cannot  be  questioned  that 


246  PHYSIOLOGICAL   DEVELOPMENT. 

this  greater  production  of  chlorophyll  next  to  the  upper 
surface  of  a  leaf,  is  directly  consequent  on  the  greater 
amount  of  light  received.  Here,  as  in  so  many  other  cases, 
we  must  regard  the  differentiation  as  in  part  due  to  direct 
equilibration  and  in  part  to  indirect  equilibration.  Fa- 
miliar facts  compel  us  to  conclude  that  from  the  beginning, 
each  individual  foliar  organ  has  undergone  a  certain  im- 
mediate adaptation  of  its  surfaces  to  the  incidence  of  light ; 
that  when  there  arose  a  mode  of  growth  which  exposed  the 
leaves  of  successive  generations  in  similar  ways,  this  im- 
mediately-produced adaptation,  ever  tending  to  be  transmitted, 
was  furthered  by  the  survival  of  individuals  inheriting  it  in 
the  greatest  degree ;  and  that  so  there  was  gradually  esta- 
blished that  difference  between  the  two  surfaces  which  each 
leaf  displays  before  it  unfolds  to  the  light,  but  which  becomes 
more  marked  when  it  has  unfolded.* 

From  the  ordinary  cases  let  us  now  pass  to  the  exceptional 
cases.  We  will  look  first  at  those  in  which  the  two  faces  of 
the  leaves  differ  but  little,  or  not  at  all — their  circumstances 
being  similar  or  equal.  Leaves  that  grow  in  approximately- 
upright  attitudes,  and  attitudes  which  do  not  maintain  the 
relative  positions  of  the  two  surfaces  with  constancy,  may  be 
expected  to  display  an  unusual  likeness  between  the  two 
surfaces  ;  and  among  them  we  see  it.  The  Grasses  may  be 
named  as  a  group  exemplifying  this  relation ;  and  if,  instead 
of  comparing  them  as  a  group  with  other  groups,  we  compare 


*  The  current  doctrine  that  chlorophyll  is  the  special  substance  concerned 
in  vegetal  assimilation,  either  as  an  agent  or  as  an  incidental  product,  must 
be  taken  with  considerable  qualification.  Besides  the  fact  that  among  the 
Alya  there  are  many  red  and  black  kinds  which  thrive  ;  and  besides  the  fact 
that  among  the  lower  Acrogens  there  are  species  which  are  purple  or  chocolate- 
coloured  ;  there  is  the  fact  that  Phsenogams  are  not  all  green.  We  have  tha 
Copper-Beech,  we  have  the  black-purple  Coleus  Verschaffeltii,  and  we  have  the 
red  variety  of  Cabbage,  which  seems  to  flourish  as  well  as  the  other  varieties. 
Chlorophyll,  then,  must  be  regarded  simply  as  the  most  general  of  the  colour- 
ing matters  found  in  those  parts  of  plants  in  which  assimilation  is  being  effected 
by  the  agency  of  light. 


THE    OUTER    TISSUES    UF    PLANTS.  247 

those  dwarf  kinds  of  them  which  spread  out  their  leaves 
horizontally,  with  the  large  aspiring  kinds,  such  as  Arundo, 
we  trace  a  like  antithesis :  in  the  one  the  contrast  of  upper 
and  under  is  very  obvious,  while  in  the  other  it  is  scarcely  to 
be  detected.  Leaves  of  various  other  Endogens  that  grow 
in  a  similar  way,  similarly  show  us  a  near  approach  to  uni- 
formity of  the  two  surfaces ;  as  instance  the  genus  Clicia, 
and  the  thinner-leaved  kinds  of  Yucca.  Where  the  con- 
trast of  upper  and  under  is  greatly  diminished  by  the  as- 
sumption of  a  rounded  or  cylindrical  form  instead  of  a  flat- 
tened form,  the  same  thing  happens.  The  genus  Kleinia 
furnishes  illustrations.  It  may  be  remarked,  too,  that 
even  within  the  limits  of  this  genus  there  are  instructive 
variations ;  for  while  in  Kleinia  ficoidcs  the  leaves,  shaped 
like  pea-pods,  are  broadest  in  a  vertical  direction,  and  have 
their  lateral  surfaces  alike  in  conditions  and  structure,  in 
other  species  the  leaves, -broader  horizontally  than  vertically, 
exhibit  unlikeness  between  the  upper  and  under  sides. 
Equally  to  the  point  is  the  evidence  furnished  by  vertically- 
growing  leaves  that  are  cylindrical,  as  those  of  Sanseviera 
cylindrica,  or  as  those  of  the  Rush-tribe  :  the  similarly-placed 
surface  has  all  around  a  similar  character.  Of 

kindred  meaning,  and  still  more  conclusive,  are  the  cases  in 
which  the  under  side  of  the  leaf,  being  more  exposed  to 
light  than  the  upper  side,  usurps  the  character  and  function 
of  the  upper  side.  If  a  common  Flag  be  pulled  to  pieces, 
it  will  be  seen  that  what  answers  to  the  face  in  other 
leaves,  forms  merely  the  inside  of  the  sheath  including  the 
younger  leaves,  and  is  obliterated  higher  up.  The  two  sur- 
faces of  the  blade  answer  to  the  two  under  halves  of  a 
leaf  that  has  been,  as  it  were,  folded  together  lengthways, 
with  the  two  halves  of  its  upper  surface  in  contact.  And 
here,  in  default  of  an  upper  surface,  the  under  surface  acquires 
its  character  and  discharges  its  function.  A  like  substitution 
occurs  in  Witscnia  corymbosa ;  and  there  are  some  of  the 
Orchids,  as  Lockhartia,  which  display  it  in  a  very  obvious  way. 


248  PHYSIOLOGICAL   DEVELOPMENT. 

When  joined  with  the  foregoing  evidence,  the  evidence 
which  another  kind  of  substitution  supplies  is  of  great 
weight.  I  refer  to  that  which  occurs  in  the  Australian 
Acacias,  already  instanced  as  throwing  light  on  morpho- 
logical changes.  In  these  plants  the  leaves  properly  so  called 
are  undeveloped,  and  the  footstalks,  flattened  out  into  folia- 
ceous  shapes,  acquire  veins  and  midribs,  and  so  far  simulate 
leaves  as  ordinarily  to  be  taken  for  them — a  fact  in  itself  of 
much  physiological  significance.  But  that  which  it  concerns 
us  especially  to  note,  is  the  absence  of  distinction  between 
the  two  faces  of  these  phyllodcs,  as  they  are  named,  and  the 
cause  of  its  absence.  These  transformed  petioles  do  not 
flatten  themselves  out  horizontally,  so  as  to  acquire  under 
and  upper  sides,  as  most  true  leaves  do ;  but  they  flatten 
themselves  out  vertically :  the  result  being  that  their  two 
sides  are  similarly  circumstanced  with  respect  to  light  and 
other  agencies;  and  there  is  consequently  nothing  to  cause 
their  differentiation.  And  then  we  find  an  analogous  case 
where  differential  conditions  arise,  and  where  some  differen- 
tiation results.  In  Oxalis  biipleurifolia,  Fig.  66,  there  is  a 
similar  flattening  out  of  the  petiole  into  a  pseudo-leaf;  but 
in  it  the  flattening  takes  place  in  the  same  plane  as  the  leaf, 
so  as  to  produce  an  under  and  an  upper  surface ;  and  here 
the  two  surfaces  of  the  pseudo-leaf  are  slightly  unlike — in 
contour  if  in  nothing  else. 

§  275.  "We  come  now  to  such  physiological  differentiations 
among  the  outer  tissues  of  plants,  as  are  displayed  in  the 
contrasts  between  foliar  organs  on  the  same  axis,  or  on 
different  axes — contrasts  between  the  seed-leaves  and  the 
leaves  subsequently  formed,  between  submerged  and  aerial 
leaves  in  certain  aquatic  plants,  between  leaves  and  bracts, 
and  between  bracts  and  sepals.  To  deal  even  briefly  with 
these  implies  information  which  even  a  professed  botanist 
would  have  to  increase  by  special  inquiries,  before  attempting 
interpretations.  Here  it  must  suffice  to  say  something 


THE    OUTER   TISSUES    OF    PLANTS.  219 

respecting  those  marked  unlikenesses  that  exist  between  the 
tissues  of  the  more  characteristic  parts  of  flowers,  and  the 
tissues  of  the  homologous  foliar  organs. 

It  was  pointed  out  in  §  196,  that  the  terminal  parts  of  a 
phaenogamic  axis  have  sundry  characters  in  common  with  such 
fronds  as  those  out  of  which  we  concluded  that  the  phaeno- 
gamic axis  has  arisen  by  integration — common  characters  of 
a  kind  to  be  expected.  In  their  simple  cellular  composition, 
comparative  want  of  chlorophyll,  and  deficiency  of  vascular 
structures,  the  undeveloped  ends  of  leaf- shoots  and  the 
developed  ends  of  flower-shoots,  approach  to  the  fronds  of  the 
simpler  Acrogens.  "We  also  noted  between  them  another 
resemblance.  It  is  said  of  the  Jungermanniacece,  that 
"  though  under  certain  circumstances  of  a  pure  green,  they 
are  inclined  to  be  shaded  with  red,  purple,  chocolate,  or  other 
tints  ; "  and  answering  to  this  we  have  the  facts  that  such 
colours  commonly  occur  in  the  terminal  folia  of  a  phaeno- 
gamic  axis  when  arrest  of  its  development  leads  to  the 
formation  of  a  flower,  and  that  very  frequently  they  are 
visible  at  the  ends  of  leaf-axes.  In  the  unfolding  parts  of 
shoots,  more  or  less  of  red,  or  copper- colour,  or  chocolate- 
colour,  may  generally  be  seen :  often  indeed  it  charac- 
terizes the  leaves  for  some  time  after  they  are  unfolded. 
Occasionally  the  traces  of  it  are  permanent ;  and,  as  in 
the  scarlet  terminal  leaves  of  Poinsetlia  pulcherrima,  we  see 
that  it  may  become,  and  continue,  extremely  conspicuous. 
The  question,  then,  now  to  be  asked,  is — has  this  colouring 
by  which  the  immature  part  of  the  phaenogamic  axis  is  cha- 
racterized, anything  to  do  with  the  colouring  of  flowers  ? 
Has  this  difference  between  undeveloped  folia  and  folia  that 
are  further  developed,  been  increased  by  natural  selection 
where  an  advantage  accrued  from  it,  until  it  has  ended  in 
the  strong  contrast  we  now  see  ?  I  think  we  may  not  irra- 
tionally infer  that  this  has  been  the  case. 

Facts,  very  numerous  and  varied,  united  to  warrant  us  in 
concluding  that  gamogcnesis  commences  where  the  forces 


250  rui'sioLOGiCAL  DEVELOPMENT 


that  conduce  to  growth  are  nearly  equilibrated  by  the  forces 
that  resist  growth  (§  78)  ;  and  the  induction  that  in  plants,  fer- 
tilized germs  are  produced  at  places  where  there  is  an  approach 
towards  this  balance,  we  found  to  be  in  harmony  with  the 
deduction  that  an  advantage  to  the  species  must  be  gained 
by  sending  off  migrating  progeny  from  points  where  nutri- 
tion is  failing.  Other  things  equal,  failure  of  nutrition 
may  be  expected  in  parts  that  have  the  most  remote  or  most 
indirect  access  to  the  materials  furnished  by  the  roots  — 
materials  that  have  to  be  carried  great  distances  by  a  very 
imperfect  apparatus.  The  ends  of  lateral  axes  are  therefore 
the  probable  points  of  fructification,  in  aggregates  of  the 
third  order  that  have  taken  to  growing  vertically.  But 
if  these  points  at  which  nutrition  is  failing,  are  also  the 
points  at  which  the  colours  inherited  from  lower  types  are 
likely  to  recur  in  more  marked  degrees  than  elsewhere  ;  then 
we  may  infer  that  the  organs  of  fructification  will  not  un- 
frequently  co-exist  with  such  colours  at  the  ends  of  such 
axes.  How  may  the  resulting  contrast  between  the  older 
fronds  and  the  fronds  next  the  germ-producing  organs  be 
increased?  If  uninterfered  with  it  would  be  likely  to  di- 
minish. .  These  traits  inherited  from  remote  ancestry,  might 
be  expected  slowly  to  fade  away.  How,  then,  is  the  intensi- 
fication of  them  to  be  explained  ? 

If  a  contrast  of  the  kind  described  favours  the  propagation  of 
a  race  in  which  it  exists,  it  will  be  maintained  and  increased  ; 
and  if  we  take  into  account  an  agency  of  which  Mr.  Darwin  has 
shown  the  great  importance  —  the  agency  of  insects  —  we  shall 
have  little  difficulty  in  understanding  how  such  a  contrast 
may  facilitate  propagation.  We  cannot,  of  course,  here  assume 
the  agency  of  insects  so  specialized  in  their  habits  as  Bees  and 
Butterflies  ;  for  their  specialized  habits  imply  the  pre-  exist- 
ence of  the  contrast  to  be  explained.  But  there  is  an  insect- 
agency  of  a  more  general  kind  which  may  be  fairly  counted 
upon  as  coming  into  action.  Various  small  Flies  and 
Beetles  wander  over  the  surfaces  of  plants  in  search  of 


THE    OUTER   TISSUES   OF    PLANTS.  251 

food.  It  is  a  legitimate  assumption  that  they  will  frequent 
most  those  parts  in  which  they  find  most  food,  or  food  most 
to  their  liking — especially  if  at  the  same  time  they  gain  the 
advantage  of  concealment.  Now  the  ends  of  axes,  formed  of 
young,  soft,  and  closely- packed  folia,  are  the  parts  which  more 
than  any  others  offer  these  several  advantages.  They  afford 
shelter  from  enemies  ;  they  frequently  contain  exuded  juices 
and  when  they  do  not,  their  tissues  are  so  tender  as  to  be 
easily  pierced  in  search  of  the  sap.  If,  then,  from  the  first,  as 
at  present,  these  ends  of  axes  have  been  favourite  haunts  of 
small  insects ;  and  if,  where  the  closely- clustered  folia  con- 
tained the  generative  organs,  the  insects  frequenting  them 
occasionally  carried  adherent  fructifying  cells  from  one  plant 
to  another,  and  so  aided  fertilization ;  it  would  follow  that 
anything  which  made  such  terminal  clusters  more  attractive 
to  such  insects,  or  more  conspicuous  to  them,  or  both,  would 
further  the  multiplication  of  the  race,  and  would  so  be  con- 
tinually increased  by  the  extra  multiplication  of  individuals 
in  which  it  was  greatest.  Here  we  find  the  clue.  This  con- 
trast of  colour  between  the  folia  next  to  the  fructifying  parts 
and  all  other  folia,  must  constantly  have  facilitated  insect- 
agency  ;  supposing  the  insects  to  have  had  the  power  of  dis- 
tinguishing between  colours.  That  Bees  and  Butterflies 
have  this  power  is  manifest :  they  may  be  watched  fly- 
ing from  flower  to  flower,  disregarding  all  other  parts 
of  the  plants.  And  if  the  less-specialized  insects  pos- 
sessed some  degree  of  such  discrimination,  then  the  initial 
contrasts  of  colour  above  described  would  be  maintained 
and  increased.  Let  such  a  connexion  be  once  established,  and 
it  must  tend  to  become  more  decided.  Insects  most  able 
to  discern  the  parts  of  plants  which  afford  what  they  seek, 
will  be  those  most  likely  to  survive  and  leave  offspring. 
Plants  presenting  most  of  the  desired  food,  and  showing  most 
clearly  where  it  lies,  will  have  their  fertilization  and  multi- 
plication furthered  in  the  greatest  degree.  And  so  the 
mutual  adaptation  will  become  ever  closer ;  while  it  is  ren- 


ZO^i  PHYSIOLOGICAL   DEVELOPMENT. 

dered  at  the  same  time  more  varied  by  the  special  require- 
ments of  the  insects  and  of  the  plants  in  each  locality,  under 
each  change  of  conditions.  Of  course,  the 

genesis  of  the  sweet  secretions  and  the  odours  of  flowers, 
has  a  parallel  interpretation.  The  simultaneous  production 
of  honey,  or  some  kindred  substance,  is  implied  above ; 
since,  unless  a  bait  co-existed  with  the  colour,  the  colour 
would  not  attract  insects,  and  would  not  be  maintained 
and  intensified  by  natural  selection.  Gums,  and  resins, 
and  balsams,  are  familiar  products  of  plants  ;  apparently, 
in  many  cases,  excreted  as  useless  matters  from  various 
parts  of  their  surfaces.  These  substances,  admitting  of 
wide  variations  in  quality,  as  they  do,  afford  opportunities 
for  the  action  of  natural  selection  wherever  any  of  them 
attractive  to  insects,  happen  to  be  produced  near  the  organs 
of  fructification.  And  this  action  of  natural  selection  once 
set  up,  may  lead  to  the  establishment  of  a  local  excretion,  to 
the  production  of  an  excretion  more  and  more  attractive,  and 
to  the  disposal  of  the  organ  containing  it  in  such  a  way  as 
most  to  facilitate  the  carrying  away  of  pollen.  Similarly 
and  simultaneously  with  odours.  Odours,  like  colours,  draw 
insects  to  flowers.  After  observing  how  Bees  come  swarming 
into  a  house  where  honey  is  largely  exposed,  or  how  Wasps 
find  their  way  into  a  shop  containing  much  ripe  fruit,  it 
cannot  be  questioned  that  insects  are  to  a  considerable  extent 
guided  by  scent.  Being  thus  sensitive  to  the  aromatic  sub- 
stances which  flowers  exhale,  they  may,  when  the  flowers  are 
in  large  masses,  be  attracted  by  them  from  distances  at  which 
the  flowers  themselves  are  invisible.  And  manifestly,  the 
flowers  which  so  attract  them  from  the  greatest  distances, 
increasing  thereby  their  chances  of  efficient  fertilization,  will 
be  most  likely  to  perpetuate  themselves.  That  is  to  say, 
survival  of  the  fittest  must  tend  to  produce  perfumes  that 
are  both  more  powerful  and  more  attractive. 

These  physiological  differentiations,  then,  which  mark  off 
the  foliar  organs  of  flowers  from  other   foliar  organs,    are 


THE    OUTER   TISSUES    OF   PLANTS.  253 

the  consequences  of  indirect  equilibration.  They  are  not 
due  to  the  immediate  actions  of  unlike  incident  forces  on 
the  parts  of  the  individual  plant ;  but  they  are  due  to  the 
actions  of  such  unlike  incident  forces  on  the  aggregate  of 
individuals,  generation  after  generation.* 

§  276.  The  unity  of  interpretation  which  we  here  find  for 
phenomena  of  such  various  orders,  could  hardly  be  found 
were  the  phenomena  otherwise  caused.  That  the  stronger 
and  the  feebler  contrasts  among  the  different  parts  of  the 
outer  tissues  in  plants,  should  so  constantly  occur  along  with 
stronger  and  feebler  contrasts  among  the  incident  forces,  is 
in  itself  weighty  evidence  that  unlike  outer  actions  have 
caused  unlike  inner  actions,  and  correspondingly-unlike  struc- 
tures ;  either  by  changing  the  functional  equilibrium  in  the 
individual,  or  by  changing  it  in  the  race,  or  by  both. 

Even  in  the  absence  of  more  direct  proof,  there  would  be 
great  significance  in  the  marked  differences  that  habitually 
exist  between  the  exposed  and  imbedded  parts  of  plants, 
between  the  stems  and  the  leaves,  and  between  the  upper  and 
under  surfaces  of  the  leaves.  The  significance  of  these  diffe- 
rences is  increased  when  we  discover  that  they  vary  in  degree 

*  This  seems  as  fit  a  place  as  any  for  noting  the  fact,  that  the  greater  part 
of  what  we  call  beauty  iu  the  organic  world,  is  in  some  way  dependent  on 
the  sexual  relation.  It  is  not  only  so  with  the  colours  and  odours  of  flowers. 
It  is  so,  too,  with  the  brilliant  plumage  of  birds,  and  with  the  songs  of  birds, 
both  of  which,  in  Mr.  Darwin's  view,  are  due  to  sexual  selection ;  and  it  is 
probable  that  the  colours  of  the  more  conspicuous  insects  are  in  part  similarly 
determined.  The  remarkable  circumstance  is,  that  these  characteristics,  which 
have  originated  by  furthering  the  production  of  the  best  offspring,  while  they 
are  naturally  those  which  render  the  organisms  possessing  them  attractive  to 
one  another,  directly  or  indirectly,  should  also  be  those  which  are  so  generally 
attractive  to  us— those  without  which  the  fields  and  woods  would  lose  half 
their  charm.  It  is  interesting,  too,  to  observe  how  the  conception  of  human 
beauty  is  in  a  considerable  degree  thus  originated.  And  the  trite  obser- 
vation that  the  element  of  beauty  which  grows  out  of  the  sexual  relation 
is  so  predominant  in  {esthetic  products — in  music,  in  the  drama,  in  fiction,  in 
poetry— gains  a  new  meaning  when  we  sec  how  deep  down  in  organic  natuie 
this  connexion  extends. 


254  PHYSIOLOGICAL    DEVELOPMENT. 

as  the  differences  in  the  conditions  vary  in  degree.  Still  greater 
becomes  the  force  of  the  evidence  on  finding  that  these 
strongly-contrasted  parts  may,  when  placed  in  one  another's 
conditions,  and  kept  in  them  from  generation  to  generation, 
permanently  assume  one  another's  functions,  and,  in  a  great 
degree,  one  another's  structures.  Even  more  conclusive 
yet  is  the  argument  rendered,  by  the  discovery  that,  where 
these  substitutions  of  function  and  structure  take  place,  the 
superinduced  modifications  differ  in  different  circumstances ; 
just  as  the  original  modifications  do.  The  fact  that  a  flattened 
stem  simulating  a  vertically-growing  leaf  has  its  two  surfaces 
alike,  while  when  it  simulates  a  horizontally-growing  leaf  its 
upper  and  under  surfaces  differ,  is  a  fact  which,  standing 
alone,  might  prove  little,  but  proves  much  when  joined  with 
all  the  other  evidence.  And  its  profound  meaning  becomes 
the  more  obvious  on  discovering  that  the  same  thing  happens 
with  petioles  when  they  usurp  leaf- functions. 

Finally,  when  we  remember  how  rapidly  analogous  modi- 
fications of  function  and  structure  arise  in  the  superficial 
tissues  of  individual  plants,  the  general  inference  can  scarcely 
be  resisted.  When  we  meet  with  so  striking  a  case  as  that 
of  the  Begonia-\ez£,  a  fragment  of  which  stuck  in  the  ground 
produces  roots  from  its  under  surface  and  leaves  from  its 
upper  surface — when  we  see  that  though,  in  this  case,  the 
typical  structure  of  the  plant  presently  begins  to  control  the 
organizing  process,  yet  the  initial  differentiations  are  set  up 
by  the  differential  actions  of  the  environment;  the  presump- 
tion becomes  extremely  strong  that  the  heterogeneities  of 
surface  which  we  have  considered,  result,  as  alleged,  directly 
or  indirectly  from  heterogeneities  in  the  incident  forces. 


CHAPTER  IV. 

DIFFERENTIATIONS  AMONG  THE  INNER  TISSUES  OF  PLANTS.* 

§  277.  In  passing  from  plants  formed  of  threads  or  thin 
lamina?,  to  plants  having  some  massiveness,  we  find  that  after 
the  external  and  internal  parts  have  become  distinguished  from 
one  another,  there  arise  dictinctions  among  the  internal  parts 
themselves,  as  well  as  among  the  external  parts  themselves : 
the  primarily-differentiated  parts  are  both  re-differentiated. 

From  types  of  very  low  organisation  illustrations  of  this 
may  be  drawn.  In  the  thinner  kinds  of  Laminaria  there 
exists  but  the  single  contrast  between  the  outer  layer  of  cells 
and  an  inner  layer ;  but  in  larger  species  of  the  same  genus, 
as  L.  digitafa,  there  are  three  unlike  layers  on  each  side  of  a 
central  layer  differing  from  them — augmentation  of  bulk  is 
accompanied  by  multiplication  of  concentric  internal  struc- 
tures, having  their  unlikenesses  obviously  related  to  unlikc- 
nesses  in  their  conditions.  In  Furcelluria  and  various  Alg& 
of  similarly  swollen  forms,  the  like  relation  may  be  traced. 

Just  indicating   the  generality  of  this  contrast,  but  not 

*  Students  of  vegetal  physiology,  familiar  with  the  controversies  respecting 
sundry  points  dealt  with  in  this  chapter,  will  probably  be  surprised  to  find 
taken  for  granted  in  it,  propositions  which  they  have  habitually  regarded  as 
open  to  doubt.  Hence  it  seems  needful  to  say  that  the  conclusions  here  set 
forth,  have  resulted  from  investigations  undertaken  for  the  purpose  of  forming 
opinions  on  several  unsettled  questions  which  I  had  to  treat,  but  which  I 
could  find  in  books  no  adequate  data  for  treating.  The  details  of  these  inves- 
tigations, and  the  entire  argument  of  which  this  chapter  is  partly  an  abstract, 
will  be  found  in  Appendix  C. 


256  PHYSIOLOGICAL   DEVELOPMENT. 

attempting  to  seek  in  these  lower  types  for  any  more  specific 
interpretation  of  it,  let  us  pass  to  the  higher  types.  The 
argument  -will  be  amply  enforced  by  the  evidence  obtained 
from  them.  We  \vill  look  first  at  the  conditions  which  they 
have  to  fulfil ;  and  then  at  the  way  in  which  the  functions 
and  structures  adapting  them  to  these  conditions  arise. 

§  278.  A  terrestrial  plant  that  grows  vertically  needs  no 
marked  modification  of  its  internal  tissues,  so  long  as  the  height 
it  reaches  is  very  small.  As  we  before  saw,  the  spiral  or 
cylindrical  rolling  up  of  a  simple  cellular  frond,  or  the  more 
bulky  growth  of  a  simple  cellular  axis,  may  give  the  requisite 
strength  ;  and  the  requisite  circulation  may  be  carried  on 
tli rough  the  unchanged  cellular  tissue.  But  in  proportion 
us  the  height  to  be  attained  and  the  mass  to  be  supported 
increase,  the  supporting  part  must  acquire  greater  bulk  or 
greater  density,  or  both  ;  and  some  modification  that  shall 
facilitate  the  transfer  of  nutritive  liquids  must  take  place. 
Hence,  in  the  inner  tissues  of  plants  we  may  expect  to  find 
that  structural  changes  answering  to  these  requirements 
become  marked,  as  the  growth  of  the  aerial  part  becomes 
great.  Facts  correspond  with  these  expectations. 

Among  the  humbler  Acrogens,  which  creep  over,  or  raise 
themselves  but  little  above,  the  surfaces  they  nourish  upon, 
there  is  scarcely  any  internal  differentiation :  the  vascular 
and  woody  structures,  if  not  in  all  cases  absolutely  un- 
represented, are  rarely  and  very  feebly  indicated.  But 
among  the  higher  Acrogens — the  Ferns  and  Lycopodiums — 
which  raise  their  fronds  to  considerable  heights,  there  are 
vascular  bundles  and  hard  tissues  like  wood;  and  by  the 
Tree-Ferns  massive  axes  are  developed.  That  the  relation 
which  thus  shows  itself  among  Cryptogams  is  habitual  among 
Phaenogams,  scarcely  needs  saying. 

Phacnogams,  however,  are  not  universally  thus  charac- 
terized iu  a  decided  way.  Besides  the  comparative  want  oi 
woody  substance  in  flowering  plants  of  humble  growth,  and 


THK    IXXEU    TISSUES   OF    PLANTS.  257 

besides  the  paucity  of  vessels  in  ordinary  water-plants,  there 
are  cases  of  much  more  marked  divergence  from,  this  typical 
internal  structure.  These  exceptional  cases  occur  under 
exceptional  conditions,  and  are  highly  instructive.  They 
are  of  two  kinds.  One  group  of  them  is  furnished 

by  certain  plants  that  are  parasitic  on  the  exposed  roots  of 
trees — parasitic  not  partially,  as  the  Mistletoe,  but  to  the 
extent  of  subsisting  wholly  on  the  sap  they  absorb.  Fungus- 
like  in  colour  and  texture,  and  having  scales  for  leaves,  these 
Balanop/'iorte  and  Rafflc^iaccce  are  recognizable  as  Phaenogams 
by  scarcely  any  other  traits  than  their  fructifications.  Along 
with  their  abortive  leaves  and  absence  of  chlorophyll,  there 
is  a  great  degradation  of  those  internal  tissues  by  which 
Phtenogams  are  commonly  distinguished.  Though  Dr. 
Hooker  has  shown  that  they  are  not,  as  some  botanists  thought, 
devoid  of  spiral  vessels ;  yet,  as  shown  by  the  mistake 
previously  made  in  classifying  them,  their  appliances  for 
circulation  are  rudimentary.  And  this  trait  goes  along  with 
a  greatly-simplified  distribution  of  nutriment.  In  the 
absence  of  leaves  there  can  be  but  little  down-current  of 
nutriment,  such  as  leaves  usually  supply  to  roots :  there 
cannot  be  much  beyond  an  upward  current  of  the  absorbed 
juices.  The  other  cases  occur  where  circulation 

is  arrested  or  checked  in  a  different  way;  namely,  in 
plants  that  are  wholly  submerged.  These  are  the  Podo- 
sfcmoncs,  which  are  aquatic  even  to  the  extent  of  flowering 
under  water.  Clothing  as  they  do  the  submerged  rocks 
in  tropical  rivers,  their  roots,  like  those  of  the  Alyw,  servo 
only  for  attachment ;  their  foliar  expansions,  frond-like  in 
shape,  are  everywhere  bathed  by  the  water;  and  their  organs 
of  fructification  never  exposed  to  the  air,  but  perhaps  aided 
in  their  functions  by  water-insects  instead  of  air-insects,  are 
the  only  marked  signs  of  kinship  to  other  Phcenogams.  Observe 
then  the  connexion  of  facts.  One  of  these  Podostcmones  needs 
iio  internal  stiffening  substance,  for  it  exists  in  a  medium  of 
its  own  specific  gravity  ;  and  having  no  unlikeness  between 


258  PHYSIOLOGICAL   DEVELOPMENT. 

the  materials  assimilated  at  its  fixed  and  its  free  ends,  it  has 
no  need  for  a  circulation — nor,  indeed,  in  the  absence  of 
evaporation  from  any  part  of  its  surface,  could  any  active 
circulation  take  place.  Here,  accordingly,  the  ordinary 
internal  structures  are  undeveloped :  though  spiral  vessels 
are  not  entirely  absent,  yet  they  are  so  rare  as  to  do  no  more 
than  verify  the  inference  of  pha3nogamic  relationship  drawn 
from  the  flowers. 

The  method  of  agreement,  the  method  of  difference,  and 
the  method  of  concomitant  variations,  thus  unite  in  proving 
A  direct  relation  between  the  demand  for  support  and  cir- 
culation, and  the  existence  of  these  vascular  woody  bundles 
which  the  higher  plants  habitually  possess.  The  question 
which  we  have  to  consider  is — Under  what  influences  are 
these  structures,  answering  to  these  requirements,  developed  ? 
How  are  these  internal  differentiations  caused  ?  The  inquiry 
may  be  conveniently  divided.  Though  the  supporting  tissues 
and  the  tissues  concerned  in  the  circulation  of  liquids  are 
closely  connected,  and  indeed  entangled,  with  one  another, 
we  may  fitly  deal  with  them  apart.  Let  us  take  first  the 
supporting  tissue. 

§  279.  Many  common-place  facts  indicate  that  the  me- 
chanical strains  to  which  upright-growing  plants  are  exposed, 
themselves  cause  increase  of  the  dense  deposits  by  which  such 
plants  are  enabled  to  resist  such  strains.  There  is  the  fact 
that  the  massiveness  of  a  tree-trunk  varies  according  to  the 
stress  habitually  put  upon  it.  If  the  contrast  between  the 
slender  stem  of  a  tree  growing  in  a  wood  and  the  bulky  stem 
of  a  kindred  tree  growing  in  the  fields,  be  ascribed  to  differ- 
ence of  nutrition  rather  than  difference  of  exposure  to  winds ; 
there  is  still  the  fact  that  a  tree  trained  against  a  wall  has  a 
less  bulky  stem  than  a  tree  of  the  same  kind  growing  un- 
supported ;  and  that  between  the  long  weak  branches  of  the 
one  and  the  stiff  ones  of  the  other  there  are  decided  contrasts. 
If  it  be  objected  that  a  tree  so  trained  and  branches  so  borne 


THE    INNVR   TISSUES   OF    PLANTS.  259 

have  relatively  less  foliage,  and  that  therefore  these  unlike- 
nesses  also  are  due  to  unlikenesscs  of  general  nutrition,  which 
may  in  part  be  true ;  there  are  still  such  cases  as  those  of 
garden  plants,  which  when  held  up  by  tying  them  to  sticks 
have  weaker  stems  than  when  they  are  unpropped,  and  sink 
down  if  their  props  are  taken  away.  Again,  there  is  the 
evidence  supplied  by  roots.  Though  the  contrast  between 
the  feeble  roots  of  a  sheltered  tree  and  the  strong  roots  of 
an  exposed  tree,  may,  like  the  contrast  of  their  stems,  be 
mainly  due  to  difference  of  nutrition,  and  therefore  supplies 
but  doubtful  evidence,  we  get  tolerably  clear  evidence  where 
trees  growing  on  inclined  rocky  surfaces,  send  into  crevices 
that  afford  little  moisture  or  nutriment,  roots  which  never- 
theless become  thick  where  they  are  so  directed  as  to  bear 
great  strains.  Suspicion  thus  raised  is  strengthened 

into  conviction  by  special  evidences  occurring  in  the  places 
where  they  are  to  be  expected.  The  Cactuses,  with  their 
succulent  growths  that  pass  into  woody  growths  slowly  and 
irregularly,  give  us  the  opportunity  of  tracing  the  conditions 
under  which  the  wood  is  formed.  Good  examples  occur  in  the 
genus  Cerem,  and  especially  iu  forms  like  C.  crenulatus.  Here, 
from  a  massive  vertically-growing  rod  of  fleshy  tissue,  two 
inches  or  more  in  diameter,  there  grow  at  intervals  lateral  rods 
similarly  bulky,  which,  quickly  curving  themselves,  take 
vertical  directions.  One  of  these  heavy  branches  puts  great 
strains  on  its  own  substance  and  that  of  the  stem  at  their 
point  of  junction  ;  and  here  both  of  them  become  brown  and 
hard,  while  they  continue  groan  and  succulent  all  around. 
Such  differentiations  may  be  traced  internally  before  they 
are  visible  on  the  surface.  If  a  joint  of  an  Opnntia  be  sliced 
through  longitudinally,  the  greater  resistance  to  the  knife 
all  around  the  narrow  neck,  indicates  there  a  larger  deposit 
of  lignin  than  elsewhere  ;  and  a  section  of  the  tissue  placed 
under  the  microscope,  exhibits  at  the  narrowest  part  a  con- 
centration of  the  woody  and  vascular  bundles.  Clear 
evidence  of  another  kind  has  been  noted  by  Mr.  Darwin,  in  the 


<J60  PHYSIOLOGICAL    DEVELOPMENT. 

organs  of  attachment  of  climbing  plants.  Speaking  of  Solan  urn 
iasminoidfs  he  says : — "  When  the  flexible  petiole  of  half- 
or  a  quarter-grown  leaf  has  clasped  any  object,  in  three  or 
four  days  it  increases  much  in  thickness,  and  after  several 
weeks  becomes  wonderfully  hard  and  rigid ;  so  that  I  could 
hardly  remove  one  from  its  support.  On  comparing  a  thin 
transverse  slice  of  this  petiole  with  one  from  the  next  or 
older  leaf  beneath,  which  had  not  clasped  anything,  its 
diameter  was  found  to  be  fully  doubled,  and  its  structure 
greatly  changed.  *  *  *  This  clasped  petiole  had  actually 
become  thicker  than  the  stem  close  beneath ;  and  this  was 
chiefly  due  to  the  greater  thickness  of  the  ring  of  wood, 
'rt'hich  presented,  both  in  transverse  and  longitudinal  sections, 
a  closely  similar  structure  in  the  petiole  and  axis.  The 
assumption  by  a  petiole  of  this  structure  is  a  singular 
morphological  fact ;  but  it  is  a  still  more  singular  physio- 
logical fact  that  so  great  a  change  should  have  been  induced 
by  the  mere  act  of  clasping  a  support." 

If  there  is  a  direct  relation  between  mechanical  stress  and 
the  formation  of  wood,  it  ought  to  explain  for  us  the  internal 
distribution  of  the  wood.  Let  us  see  whether  it  does  this. 

When  seeking  in  mechanical  actions  and  reactions  the 
cause  of  that  indurated  structure  which  forms  the  verte- 
brate axis  (§§  254-7),  it  was  pointed  out  that  in  a  transversely- 
strained  mass,  the  greatest  pressures  and  tensions  are  thrown 
on  the  molecules  of  the  concave  and  convex  surfaces.  Hence, 
supposing  the  transversely-strained  mass  to  be  a  cylinder, 
bent  backwards  and  forwards  not  in  one  plane  but  now  in 
this  plane  and  now  in  that,  its  peripheral  layers  will  be 
those  on  which  the  greatest  stress  falls.  An  ordinary 
exogenous  axis  is  such  a  cylinder  so  strained.  The  main- 
tenance of  its  attitude  either  as  a  lateral  shoot  or  a  vertical 
shoot,  implies  subjection  to  the  bendings  caused  by  its  own 
weight  and  by  the  ever-varving  wind.  These  bendings 
imply  tensions  and  pressures  falling  most  severely  first  on 
one  side  of  its  outer  layers  and  then  on  another.  And  if  tlio 


TISSUES    OF    fl.ANTS.  261 

dense  substance  able  to  resist  these  tensions  and  pressures  is 
deposited  most  where  they  are  greatest,  we  ought  to  find  it 
taking  the  shape  of  a  cylindrical  casing.  This  is  just  what 
we  do  find.  Oil  cutting  across  a  shoot  in  course  of  formation, 
we  see  its  central  space  either  unoccupied  or  occupied  only 
by  soft  tissue.  That  the  layer  of  hard  tissue  surrounding 
this  is  not  the  outermost  layer,  is  true  :  there  lies  beyond  it 
the  cambium  layer,  from  which  it  is  formed.  But  outside 
of  the  cambium  there  is  another  layer  of  dense  tissue,  the 
liber,  having  frequently  a  tenacity  greater  even  than  that  of 
the  wood — a  layer  which,  while  it  protects  the  cambium  and 
offers  additional  resistance  to  the  transverse  strain,  admits  of 
being  fissured  as  fast  as  the  cylinder  of  wood  thickens.  That 
is  to  say,  the  deposit  of  resisting  substance  is  as  completely 
peripheral  as  the  exogenous  mode  of  growth  permits.  So, 
too,  in  general  arrangement  is  it  with  the  endogenous  stem. 
Different  as  is  here  the  mode  of  growth,  and  different  as  is 
the  internal  structure,  there  }ret  holds  the  same  general  dis- 
tribution of  tissues,  answering  to  the  same  mechanical  con- 
ditions. The  vascular  woody  bundles,  more  abundant  towards 
the  outside  of  the  stem  than  near  the  centre,  produce  a  harder 
casing  surrounding  a  softer  core.  In  the  supporting 

structures  of  leaves  we  find  significant  deviations  from  this 
arrangement.  While  axes  are  on  the  average  exposed  to 
equal  strains  on  all  sides,  most  leaves,  spreading  out  their 
surfaces  horizontally,  have  their  petioles  subject  to  strains 
that  are  not  alike  in  all  directions  ;  and  in  them  the  hard 
tissue  is  differently  arranged.  Its  transverse  section  is 
not  ring-shaped  but  crescent-shaped :  the  two  horns  being 
directed  towards  the  upper  surface  of  the  petiole.  That  this 
arrangement  is  one  which  answers  to  the  mechanical  con- 
ditions, is  not  easy  to  demonstrate  :  we  must  satisfy  ourselves 
by  noting  that  here,  where  the  distribution  of  forces  is 
different,  the  distribution  of  resisting  tissue  is  different.  And 
then,  showing  conclusively  the  connexion  between  these  differ- 
ences, we  have  the  fact  that  in  petioles  growing  vertically 


2G2  PHYSIOLOGICAL    DEVELOPMENT. 

Qnd  supporting  peltate  leaves — petioles  which  are  therefore 
subject  to  equal  transverse  strains  on  all  sides — the  vascular 
bundles  are  arranged  cylindrically,  as  in  axes. 

Such,  then,  are  some  of  the  reasons  for  concluding  that  the 
development  of  the  supporting  tissue  in  plants,  is  caused  by 
the  incident  forces  which  this  tissue  has  to  resist.  The 
individuals  in  which  this  direct  balancing  of  inner  and  outer 
actions  progresses  most  favourably,  are  those  which,  other 
things  equal,  are  most  likely  to  prosper ;  and  by  habitual 
survival  of  the  fittest,  there  is  established  a  systematic  and 
constant  distribution  of  a  deposit  adapted  to  the  circumstances 
of  each  type. 

§  280.  The  function  of  circulation  may  now  be  dealt  with. 
We  have  to  consider  here  by  what  structures  this  is  dis- 
charged ;  and  what  connexion  exists  between  the  demand 
for  them  and  the  genesis  of  them. 

The  contrast  between  the  rates  at  which  a  dye  passes 
through  simple  cellular  tissue  and  cellular  tissue  of  which  the 
units  have  been  elongated,  indicates  one  of  the  structural 
changes  required  to  facilitate  circulation.  If  placed  with  its 
cut  surface  in  a  coloured  liquid,  the  parenchyma  of  a  potato 
or  the  medullary  mass  of  a  cabbage-stalk,  will  absorb  the 
liquid  with  extreme  slowness  ;  but  if  the  stalk  of  a  fungus  bo 
similarly  placed,  the  liquid  runs  up  it,  and  especially  up  its 
loose  central  substance,  very  quickly.  On  comparing  the 
tissues  which  thus  behave  so  differently,  we  find  that  whereas 
in  the  one  case  the  component  cells,  packed  close  together, 
have  deviated  from  their  primitive  sphericity  only  as  much  as 
mutual  pressure  necessitates,  in  the  other  case,  they  are  drawn 
out  into  long  tubules  with  narrow  spaces  among  thorn — the 
greatest  dimensions  of  the  tubules  and  the  spaces  being  in  the 
direction  which  the  dye  takes  so  rapidly.  That  which  we 
should  infer,  then,  from  the  laws  of  capillary  action,  is 
experimentally  shown  :  liquid  moving  through  tissues  follows 
the  lines  in  which  the  elements  of  the  tissues  are  most 


THE    INNER    TISSUES    OF    PLANTS.  263 

elongated.  It  does  this  for  two  reasons.  That  narrowing  ol 
the  cells  and  intercellular  spaces  which  accompanies  their 
elongation,  facilitates  capillarity  ;  and  at  the  same  time  fewer 
of  the  septa  formed  by  the  joined  ends  of  the  cells  have  to  be 
passed  through  in  a  given  distance.  Hence  the 

general  fact  that  the  establishment  of  a  rudimentary  vascular 
system,  is  the  formation  of  bundles  of  cells  lengthened  in  the 
direction  which  the  liquid  is  to  take.  This  we  see  very 
obviously  among  the  lower  Acrogens.  In  one  of  the  lichen- 
like  Liverworts,  the  veins  which,  branching  through  its 
frond,  serve  as  communications  with  its  scattered  rootlets,  are 
formed  of  cells  longer  than  those  composing  the  general  tissue 
of  the  frond  :  the  lengths  of  these  cells  corresponding  in  their 
directions  with  the  lengths  of  the  veins.  So,  too,  is  it 
with  the  midribs  of  such  fronds  as  assume  more  definite 
shapes  ;  and  so,  too,  is  it  with  the  creeping  stems  which 
unite  many  such  fronds.  That  is  to  say,  the  current  which 
sets  towards  the  growing  part  from  the  part  which  supplies 
the  materials  for  growth,  sets  through  a  portion  of  the  tissues 
composed  of  units  that  are  longer  in  the  line  of  the  current 
than  at  right  angles  to  that  line.  The  like  is  true 

of  Phaenogams.  Omitting  all  other  characteristics  of  those 
parts  of  them  through  which  chiefly  the  currents  of  sap 
flow,  we  find  the  uniform  fact  to  be  that  they  consist  of  cells 
and  intercellular  spaces  distinguished  from  others  by  their 
lengths.  It  is  thus  with  veins,  and  midribs,  and  petioles; 
and  if  we  wish  proof  that  it  is  thus  with  stems,  we  have  but 
to  observe  the  course  taken  by  a  coloured  solution  into  which 
a  stem  is  inserted. 

What  is  the  original  cause  of  this  differentiation  ?  Is  it 
possible  that  this  modification  of  cell-structure  which  favours 
the  transfer  of  liquid  towards  each  place  of  demand,  is  itself 
caused  by  the  current  which  the  demand  sets  up  ?  Does  the 
stream  make  its  own  channel  ?  There  are  various  reasons 
for  thinking  that  it  does.  In  the  first  place,  the  simplest  and 
earliest  channels,  such  as  we  see  in  the  Liverworts,  do  not 
VOL.  II.  12 


Xi64  PHYSIOLOGICAL    DEVELOPMENT. 

develop  in  any  systematic  way,  but  branch  out  irregularly, 
following  everywhere  the  irregular  lobes  of  the  frond  as 
these  spread ;  and  on  examining  under  a  magnifier  the  places 
at  wnich  the  veins  are  lost  in  the  cellular  tissue,  it  will  be 
seen  that  the  cells  are  there  slightly  longer  than  those 
around :  suggesting  that  the  lengthening  of  them  which 
produces  an  extension  of  the  veins,  takes  place  as  fast  as 
the  growth  of  the  tissue  beyond  causes  a  current  to  pass 
through  them.  In  the  second  place,  a  disappearance  of  the 
granular  contents  of  these  cells  accompanies  their  union 
into  a  vein — a  result  which  the  transmission  of  a  current 
may  not  improbably  bring  about.  But  be  the  special  causes 
of  this  differentiation  what  they  may,  the  evidence  favours 
very  much  the  conclusion  that  the  general  cause  is  the 
setting  up  of  a  current  towards  a  place  where  the  sap  is 
being  consumed.  In  the  histological  development 

of  the  higher  plants  we  find  confirmation  The  more 
finished  distributing  canals  in  Phaenogams  are  formed  of  cells 
previously  lengthened.  At  parts  of  which  the  typical  struc- 
ture is  fixed,  and  the  development  direct,  this  fact  is  not  easy  to 
trace  ;  the  cells  rapidly  take  their  fibrous  structures  in  antici- 
pation of  their  pre-determined  functions.  But  in  places 
where  new  vessels  are  required  in  adaptation  to  a  modify- 
ing growth,  we  may  clearly  trace  this  succession.  The 
swelling  root  of  a  turnip,  continually  having  its  vascular 
system  further  developed,  and  the  component  vessels 
lengthened  as  well  as  multiplied,  gives  us  an  opportunity  of 
watching  the  process.  In  it  we  see  that  the  reticulated  cells 
which  unite  to  form  ducts,  arise  in  the  midst  of  bundles  of 
cells  that  have  previously  become  elongated,  and  that  they 
arise  by  transformation  of  such  elongated  cells;  and  we 
also  see  that  these  bundles  of  elongated  cells  have  an 
arrangement  quite  suggestive  of  their  formation  by  passing 
currents. 

Are  there  grounds  for  thinking  that  these  further  trans- 
formations  by  which   strings   of  elongated  cells  pass    into 


THE    1NXER   TISSUES   OF    PLANTS.  265 

vessels  lined  with  spiral,  annular,  reticulated,  or  other 
frameworks,  are  also  in  any  way  determined  by  the  currents 
of  sap  carried  ?  There  are  some  such  grounds. 

As  just  indicated,  the  only  places  where  we  may  look 
for  evidence  with  any  rational  hope  of  finding  it,  are 
places  where  some  local  requirement  for  vessels  has  arisen, 
in  consequence  of  some  local  development  which  the  type 
does  not  involve.  In  these  cases  we  find  such  evidence. 
Good  illustrations  occur  in  those  genera  of  the  Cactacece, 
which  simulate  leaves,  like  Epiphyllnm  and  PhylloQUCtus. 
A  branch  of  one  of  these  is  outlined  in  Fig.  256.  As  before 
explained,  this  is  a  flattened  axis ;  and  the  notches  along 
its  edges  are  the  seats  of  the  axillary  buds.  Most  of  these 
axillary  buds  are  arrested;  but  occasionally  one  of  them 
grows.  Now  if,  taking  an  Spiphyltum-f&SOOt  which  bears 
a  lateral  shoot,  we  compare  the  parts  of  it  that  are  near 
the  abortive  axillary  buds  with  the  part  that  is  near  the 
developed  axillary  bud,  we  find  a  conspicuous  difference. 
In  the  neighbourhood  of  an  abortive  axillary  bud  there 
is  no  external  sign  of  any  internal  differentiation  ;  and  on 
holding  up  the  branch  against  the  light,  the  uniform  trans- 
luceiicy  shows  that  there  is  no  greater  amount  of  dense 
tissue  near  it  than  in  other  parts  of  the  succulent  mass. 
But  where  an  axillary  bud  has  developed,  a  prominent  rounded 
ridge  joins  the  midrib  of  the  lateral  branch  with  the  midrib 
of  the  parent  branch.  In  the  midst  of  this  rounded  ridge 
an  opaque  core  may  be  seen.  And  on  cutting  through  it,  this 
opaque  core  proves  full  of  vascular  bundles  imbedded  in 
woody  deposits.  Clearly,  these  clusters  of  vessels  imply 
transformations  of  the  tissues,  caused  by  the  passage  of 
increased  currents  of  sap.  The  vessels  were  not  there  when 
the  axillary  bud  was  formed ;  they  would  not  have  de- 
veloped had  the  axillary  bud  proved  abortive  ;  but  they 
arise  as  fast  as  growth  of  the  axillary  bud  draws  the  sap 
along  the  lines  in  which  they  lie.  Verification  is  obtained 
by  examining  the  internal  structures.  If  longitudinal 


266  PHYSIOLOGICAL    DEVELOPMENT. 

sections  be  made  through  a  growing  bud  of  Opuntia  or 
Cerctis,  it  will  be  found  that  the  vessels  in  course  of  for- 
mation converge  towards  the  point  of  growth,  as  they  would 
do  if  the  sap-currents  determined  their  formation ;  that 
they  are  most  developed  near  their  place  of  convergence, 
which  they  also  would  be  if  so  produced  ;  and  that  their 
terminations  in  the  tissue  of  the  parent  shoot  are  partially- 
formed  lines  of  irregular  fibrous  cells,  like  those  out  of 
which  the  vessels  of  a  leaf  or  bud  are  developed. 

Concluding,  then,  that  sap-vessels  arise  along  the  lines  of 
least  resistance,  through  which  currents  are  drawn  or  forced, 
the  question  to  be  asked  is — What  physical  process  produces 
them  ?  Their  component  cells,  united  end  to  end  more  or  less 
irregularly  in  ways  determined  by  their  original  positions, 
form  a  channel  much  more  permeable,  both  longitudinally 
and  laterallv,  than  the  tissue  around.  How  is  this  greater 
permeability  caused  ?  The  idea,  first  propounded 

I  believe  by  Wolff,  that  the  adjoined  ends  of  the  cells  are 
perforated  or  destroyed  by  the  passing  current,  is  one  for 
which  much  is  to  be  said.  Whether  these  septa  are  dissolved 
by  the  liquids  they  transmit,  or  whether  they  are  burst  by  those 
sudden  gushes  which,  as  we  shall  hereafter  see,  must  frequently 
take  place  along  these  canals,  needs  not  be  discussed  :  it  is 
sufficient  for  us  that  the  septa  do,  in  many  cases,  disappear, 
leaving  internal  ridges  showing  their  positions  ;  and,  in  other 
cases,  become  extremely  porous.  Though  it  is  manifest  that 
this  is  not  the  process  of  vascular  development  in  tissues  that 
unfold  after  pre-determined  types,  since,  in  these,  the  dehi- 
scences  or  perforations  of  septa  occur  before  such  direct 
actions  can  have  come  into  play ;  yet  it  is  still  possible 
that  the  disappearances  of  septa  which  now  arise  bv  repe- 
tition of  the  type  were  established  in  the  type  by  such 
direct  actions.  Be  this  as  it  may,  however,  a 

simultaneous    change   undergone    by    these    longitudinall}-- 
unitcd    cells  must   be   otherwise  caused.     Frame-works   ar 
formed  in  them—  frame- works  which,  closely  fitting  their  inner 


THE    INSLH   TISSUES    OF   PLANTS.  2G7 

surfaces,  may  consist  either  of  successive  rings,  or  continuous 
spiral  threads,  or  networks,  or  structures  between  spirals  and 
networks,  or  networks  with  openings  so  far  diminished  that  the 
cells  containing  them  are  distinguished  as  fenestrated.  Their 
differences  omitted,  however,  these  structures  have  the  common 
character  that,  while  supporting  the  coats  of  the  vessels  and 
serving  to  restore  their  diameters  after  they  have  been  com- 
pressed, they  also  give  special  facilities  for  the  passage  of 
liquids,  both  through  the  sides  of  the  transformed  cells  and 
through  their  united  ends,  where  these  are  not  destroyed. 
For  one  of  these  internal  frame- works  is  not,  as  usually  stated, 
produced  by  the  deposition  of  substance  on  the  cell-mem- 
brane, in  the  shape  which  the  frame-work  eventually  assumes. 
Were  it  so,  this  frame- work  would  have  a  thickness  additional 
to  that  of  the  cell- wall  as  previously  existing,  which  it  has  not. 
On  comparing  one  of  these  cells  longitudinally  cut  through, 
with  an  adjacent  cell  of  the  kind  to  which  it  was  originally 
similar,  we  see  that  over  every  opening  in  the  frame- work,  the 
Avail  of  the  cell  is  far  thinner  than  the  walls  of  the  adjacent 
cells:  the  cell-membrane  at  each  of  these  openings  being  quite 
bare,  instead  of  being-,  as  in  adjacent  cells,  covered  by  a  layer  of 
deposit.  Hence  this  transformation  of  cells  into  sap-channels, 
is  in  part  the  arrangement  or  re-arrangement  of  their  sub- 
stance in  such  ways  as  greatly  to  diminish  the  resistance  to 
the  passage  of  liquid,  both  longitudinally  and  laterally. 

To  attempt  any  physical  interpretation  of  this  change 
is  scarcely  safe :  the  conditions  are  so  complex.  There  are 
many  reasons  for  suspecting,  however,  that  it  arises  from  a 
vacuolation  of  the  substance  deposited  on  the  cell  wall.  It 
rapidly  deposited,  as  it  is  likely  to  be  along  lines  where  sap 
is  freely  supplied,  this  may,  in  passing  from  the  state  of  a 
soluble  colloid  to  that  of  an  insoluble  colloid,  so  contract  as  to 
leave  uncovered  spaces  on  the  cell-membrane ;  and  this 
change,  originally  consequent  on  a  physico-chemical  action, 
may  be  so  maintained  and  utilized  by  natural  selection,  as  to 
result  in  structures  of  a  definite  kind,  regularly  formed  in 


263  PHYSIOLOGICAL    DEVELOPMENT. 

growing  parts  in  anticipation,  of  functions  to  be  afterwards 
discharged.  But,  without  alleging  any  special  cause  for  this 
metamorphosis,  there  is  good  evidence  that  it  is  in  some  way 
consequent  upon  the  carrying  of  sap.  If  we  examine  tissues 
such  as  that  in  the  interior  of  a  growing  turnip  that  has 
not  yet  become  stringy,  we  may,  in  the  first  place,  find 
bundles  of  elongated  cells  not  having  yet  developed  in  them 
those  fenestrated  or  reticulated  structures  by  which  the  ducts 
are  eventually  characterized.  Along  the  centres  of  adjacent 
bundles  we  may  find  incomplete  lines  of  such  cells — some  that 
are  partially  or  wholly  transformed,  with  some  between  them 
that  are  not  transformed.  In  other  bundles,  completed  chains 
of  such  transformed  cells  are  visible.  And  then,  in  still 
older  bundles,  there  are  several  complete  chains  running  side 
\)y  side.  All  which  facts  imply  a  metamorphosis  of  the 
elongated  cells,  caused  bv  the  continued  action  of  the  currents 
carried. 

'§  281.  Here,  however,  presents  itself  a  further  problem. 
Taking  it  as  manifest  that  there  is  a  typical  distribution  of 
supporting  tissue  adapted  to  meet  the  mechanical  strains  a 
plant  is  exposed  to  by  its  typical  mode  of  growth,  and  also 
that  there  goes  on  special  adaptation  of  the  supporting  tissue 
to  the  special  strains  the  individual  plant  has  to  bear ;  and 
taking  it  as  tolerably  evident  that  the  sap  channels  are 
originally  determined  by  the  passage  of  currents  along  lines 
of  least  resistance;  there  still  remains  the  ultimate  question — 
Through  what  physical  actions  are  established  these  general 
and  special  adjustments  of  supporting  tissue  to  the  strains 
borne,  and  these  distributions  of  nutritive  liquid  required  to 
nuike  possible  such  adjustments  ?  Clearly,  if  the  external 
actions  produce  internal  reactions;  and  if  this  play  of  actions 
and  reactions  results  in  a  balancing  of  the  strains  by  the 
resistances ;  we  may  rationally  suspect  that  the  incident 
forces  are  directly  conducive  to  the  structural  changes  by 
which  they  are  met.  Let  us  consider  how  they  must  work. 


THE    1XXER   TISSUES    OF   PLANTS.  269 

When  any  part  of  a  plant  is  bent  by  the  wind,  the  tissues 
on  its  convex  surface  are  subject  to  longitudinal  tension,  and 
these  extended  outer  layers  cornjress  the  layers  beneath 
them.  Such  of  the  vessels  or  canals  in  these  subjacent  layers 
as  contain  sap,  must  have  some  of  this  sap  expelled.  Part  of 
it  will  be  squeezed  through  the  more  or  less  porous  walls  of 
the  canals  into  the  surrounding  tissue,  thus  supplying  it 
with  assimilable  materials ;  while  part  of  it,  and  probably 
the  larger  part,  will  be  thrust  along  the  canals  longitudinally 
upwards  and  downwards.  When  the  branch  or  twig  or  leaf- 
stalk recoils,  these  vessels,  relieved  from  pressure,  expand  to 
their  original  diameters.  As  they  expand,  the  sap  rushes 
back  into  them  from  above  and  below.  In  whichever  of 
these  directions  least  has  been  expelled  by  the  compression, 
from  that  direction  most  must  return  during  the  dilation  ; 
seeing  that  the  force  which  more  efficiently  resisted  the 
thrusting  back  of  the  sap  is  the  same  force  which  urges  it 
into  the  expanded  vessels  again,  when  they  are  relieved  from 
pressure.  At  the  next  bend  of  the  part  a  further  portion  of 
sap  will  be  squeezed  out,  and  a  further  portion  thrust  for- 
wards along  the  vessels.  This  rude  pumping  process  thus 
serves  for  propelling  the  sap  to  heights  which  it  could  not 
reach  by  capillary  action,  at  the  same  time  that  it  incident- 
ally serves  to  feed  the  parts  in  which  it  takes  place.  It 
strengthens  them,  too,  just  in  proportion  to  the  stress  to  be 
borne  ;  since  the  more  severe  and  the  more  repeated  the 
^trains,  the  greater  must  be  the  exudation  of  sap  from  the 
vessels  or  ducts  into  the  surrounding  tissue,  and  the  greater 
the  thickening  of  this  tissue  by  secondary  deposits.  By 

this  same  action  the  movement  of  the  sap  is  determined 
either  upwards  or  downwards,  according  to  the  conditions. 
While  the  leaves  are  active  and  evaporation  is  going  on  from 
them,  these  oscillations  of  the  branches  and  petioles  urge 
forward  the  sap  into  them  ;  because  so  long  as  the  vessels  of 
the  leaves  are  being  emptied,  the  sap  in  the  compressed 
vessels  of  the  oscillating  parts  will  meet  with  less  resistance 


27Q  PHYSIOLOGICAL    DEVELOPMENT. 

hi  the  direction  of  the  leaves  than  in  the  opposite  direction. 
But  when  evaporation  ceases  at  night,  this  will  no  longer  be 
the  case.  The  sap  drawn  to  the  oscillating  parts,  to  supply 
the  place  of  the  exuded  sap,  must  come  from  the  directions 
of  least  resistance.  A  slight  breeze  will  bring  it  back  from 
the  leaves  into  the  gently-swaying  twigs,  a  stronger  breeze 
into  the  bending  branches,  a  gale  into  the  strained  stem  and 
roots — roots  in  which  longitudinal  tension  produces,  in 
another  way,  the  same  effects  that  transverse  tension  does  in 
the  branches. 

Two  possible  misinterpretations  must  be  guarded  against. 
It  must  not  be  supposed  that  this  force-pump  action  causes 
movement  of  the  sap  to \vards  one  point  rather  than 
another :  it  is  simply  an  aid  to  its  movement.  From  the 
stock  of  sap  distributed  through  the  plant,  more  or  less  is 
everywhere  being  abstracted — here  by  evaporation  ;  here  by 
the  unfolding  of  the  parts  into  their  typical  shapes ;  here  by 
both.  The  result  is  a  tension  on  the  contained  liquid  columns, 
that  is  greatest  now  in  this  direction  and  now  in  that.  This 
tension  it  is  which  must  be  regarded  as  the  force  that 
determines  the  current  upwards  or  downwards;  and  all  which 
the  mechanical  actions  do  is  to  facilitate  the  transfer  to  the 
places  of  greatest  demand.  Hence  it  happens  that  in  a  plant 
prevented  from  oscillating,  but  having  a  typical  tendency  to 
assume  a  certain  height  and  bulk,  the  demands  set  up  by  its 
unfolding  parts  will  still  cause  currents ;  and  there  will  still 
be  alternate  ascents  and  descents,  according  as  the  varying 
conditions  change  the  direction  of  greatest  demand — the 
only  difference  being,  that  in  the  absence  of  oscillations  the 
the  growth  will  be  less  vigorous.  Similarly,  it  must 

not  be  supposed  that  mechanical  actions  are  here  alleged  to  be 
the  sole  causes  of  wood-formation  in  the  individual  plant.  The 
tendency  of  the  individual  plant  to  form  wood  at  places  where 
wood  has  been  habitually  formed  by  ancestral  plants,  is 
manifestly  a  cause,  and,  indeed,  the  chief  cause.  In  this,  as 
in  all  other  cases,  inherited  structures  repeat  themselves 


II1E   LNXER   TISSUES   OF    PLANTS.  271 

irrespective  of  the  circumstances  of  the  individual :  absence 
of  the  appropriate  conditions  resulting  simply  in  imperfect 
repetition  of  the  structures.  Hence  the  fact  that  in  trained 
trees  and  hothouse  shrubs,  dense  substance  is  still  largely 
deposited ;  though  not  so  largely  as  where  the  normal  me- 
chanical strains  have  acted.  Hence,  too,  the  fact,  that 
in  such  plants  as  the  Elephants -foot  or  the  Welwitschia 
mirabilis,  which  for  untold  generations  can  have  undergone 
no  oscillations,  there  is  an  extensive  formation  of  wood 
(though  not  to  any  considerable  height  above  the  ground),  in 
repetition  of  an  ancestral  type :  natural  selection  having 
here  maintained  the  habit  as  securing  some  other  advantage 
than  that  of  support. 

Still,  it  must  be  borne  in  mind  that  though  intermittent 
mechanical  strains  cannot  be  assigned  as  the  direct  causes  of 
these  internal  differentiations  in  plants  that  are  artificially 
sheltered  or  supported,  they  are  assignable  as  the  indirect 
causes ;  since  the  inherited  structures,  repeated  apart  from 
such  strains,  are  themselves  interpretable  as  accumulated 
results  of  such  strains  acting  on  successive  generations  of 
ancestral  plants.  This  will  become  clear  on  combining  the 
several  threads  of  the  argument  and  bringing  it  to  a  close, 
which  we  may  now  do. 

§  282.  To  put  the  co-operative  actions  in  their  actual  order, 
would  require  us  to  consider  them  as  working  on  individuals 
small  modifications  that  become  conspicuous  and  definite 
only  by  inheritance  and  gradual  increase  ;  but  it  will  aid  our 
comprehension  without  leading  us  into  error,  if  we  suppose  the 
whole  process  resumed  in  a  single  continuously-existing  plant. 

As  the  plant  erects  the  integrated  series  of  fronds  whose 
united  parts  form  its  rudimentary  axis,  the  increasing  area 
of  frond-surface  exposed  to  the  sun's  rays  entails  an  increasing 
draught  upon  the  liquids  contained  in  the  rudimentary 
axis.  The  currents  of  sap  so  produced,  once  established  along 
certain  lines  of  cells  that  offer  least  resistance,  render  them 


272  PHYSIOLOGICAL    DEVELOPMENT. 

by  their  continuous  passage  more  and  more  permeable.  This 
establishment  of  channels  is  aided  by  the  wind.  Each  bend 
produced  by  it  while  yet  the  tissue  is  undifferentiated, 
squeezes  towards  the  place  of  growth  and  evaporation  the 
liquids  that  are  passing  by  osmose  from  cell  to  cell ;  and 
when  the  lines  of  movement  become  defined,  each  bend  helps, 
by  forcing  the  liquid  along  these  lines,  to  remove  obstructions 
and  make  continuous  canals.  As  fast  as  this  transfer  of  sap 
is  facilitated,  so  fastis  the  plant  enabled  further  to  raise  itself, 
and  add  to  its  assimilating  surfaces ;  and  so  fast  do  the 
"transverse  strains,  becoming  greater,  give  more  efficient 
aid.  The  channels  thus  formed  can  be  neither  in  the 
centre  of  the  rudimentary  axis  nor  at  its  surface  ;  for  at 
neither  of  these  places  can  the  transverse  strains  produce 
any  considerable  compressions.  They  must  arise  along  a  tract 
between  the  outside  of  the  axis  and  its  core — a  tract  along 
which  there  occur  the  severest  squeezes  between  the  ex- 
tended outer  layers  and  the  internal  mass.  Just  that  dis- 
tribution which  we  find,  is  the  distribution  which  these  me- 
chanical actions  tend  to  establish. 

As  the  plant  gains  in  height,  and  as  the  mass  of  its  foliage 
accumulates,  the  strains  thrown  upon  its  axis,  and  especially 
the  lower  part  of  its  axis,  rapidly  increase.  Supposing  the 
forms  to  remain  similar,  the  strains  must  increase  in  the  ratio 
of  the  cubes  of  the  dimensions  ;  or  even  in  a  somewhat  higher 
ratio.  One  consequence  must  be,  that  ihe  compressions  to 
which  the  vessels  at  the  lower  part  of  the  stem  are  subject, 
become  greater  as  fast  as  the  height  to  which  the  sap  has  to 
be  raised  becomes  greater ;  and  another  consequence  must  be, 
that  the  local  exudation  of  sap  produced  by  the  pressure  is 
proportionately  augmented.  Hence  the  materials  for  nutri- 
tion of  the  surrounding  tissues  being  there  supplied  more 
abundantly,  we  may  expect  thickening  of  the  surrounding 
tissues  to  show  itself  there  first :  in  other  -words,  wood 
will  be  formed  round  the  vessels  of  the  lower  part  of 
the  stem.  The  resulting  greater  ability  of  this  lower 


THE    INNER   TISSUES    OF    PLANTS.  273 

part  of  the  stem  to  bear  strains,  renders  possible  an  increase 
of  height ;  and  while  after  an  increase  of  height  the  lowest 
part  becomes  still  further  strained,  and  still  further  thickens, 
the  part  above  it,  exposed  to  like  actions,  undergoes  a  like 
thickening.  This  induration,  while  it  spreads  upwards, 
also  spreads  outwards.  As  fast  as  the  rude  cylinder  of  dense 
matter  formed  in  this  way,  begins  to  inclose  the  original 
vessels,  it  begins  to  play  the  part  of  a  resistant  mass,  between 
which  and  the  outer  layers  the  greatest  compression  occurs 
at  each  bend.  While,  therefore,  the  original  vessels  become 
useless,  the  peripheral  cells  of  the  developing  wood  become 
those  which  have  their  liquid  contents  squeezed  out  longitu- 
dinally and  laterally  with  the  greatest  force;  and,  consequently, 
amid  them  are  formed  new  sap-channels,  from  which  there  ia 
the  most  active  local  exudation,  producing  the  greatest 
deposit  of  dense  matter. 

Thus  fusing  together,  as  it  were,  the  individualities  of 
successive  generations  of  plants,  and  letting  that  facilitation 
of  the  process  which  natural  selection  has  all  along  given, 
be  represented  by  the  most  favourable  working  together  of 
these  mechanical  processes,  we  are  enabled  to  interpret 
the  leading  internal  differentiations  of  plants  as  consequent 
on  a  direct  equilibration  between  inner  and  outer  forces. 
Here,  indeed,  we  see  illustrated  in  a  way  more  than  usually 
easy  to  follow,  the  eventual  balancing  of  outer  actions  by 
inner  reactions.  The  relation  between  the  demand  for  liquid 
and  the  formation  of  channels  that  supply  liquid,  as  well 
as  that  between  the  incidence  of  strains  and  the  deposit 
of  substance  that  resists  strains,  are  among  the  clearest  special 
examples  of  the  general  truth  that  the  moving  equilibrium 
of  an  organism,  if  not  overthrown  by  an  incident  force,  must 
eventually  be  adjusted  to  it. 

The  processes  here  traced  out  are,  of  course,  not  to  be 
taken  as  the  only  differentiating  processes  to  which  the 
inner  tissues  of  plants  have  been  subject.  Besides  the  chief 
changes  we  have  considered,  various  less  conspicuous  changes 


274  PHYSIOLOGICAL   DEVELOPMENT. 

have  taken  place.  These  must  be  passed  over  as  arising 
in  ways  too  involved  to  admit  of  npecific  interpreta- 
tions ;  even  supposing  them  to  have  been  produced  by 
causes  of  the  kind  assigned.  But  the  probability,  or 
rather  indeed  the  certainty,  is,  that  some  of  them  have  not 
been  so  produced.  Here,  as  in  nearly  all  other  cases,  in- 
diect  requilibration  has  worked  in  aid  of  direct  equilibration  ; 
and  in  many  cases  indirect  equilibration  has  been  the  sole 
agency.  Besides  ascribing  to  natural  selection  the  rise  of 
various  internal  modifications  of  other  classes  than  those 
above  treated,  we  must  ascribe  some  even  of  these  to  natural 
selection.  It  is  so  with  the  dense  deposits  which  form 
thorns  and  the  shells  of  nuts:  these  cannot  have  resulted 
from  any  inner  reactions  immediately  called  forth  by  outer 
actions ;  but  must  have  resulted  mediately  through  the  effects 
of  such  outer  actions  on  the  species.  Let  it  be  understood, 
therefore,  that  the  differentiations  to  which  the  foregoing 
interpretation  applies,  are  only  those  most  conspicuous  ones 
which  are  directly  related  to  the  most  conspicuous  in- 
cident forces.  They  must  be  taken  as  instances  on  the 
strength  of  which  we  may  conclude  that  other  internal 
differentiations  have  had  a  natural  genesis,  though  in  ways 
that  we  cannot  trace. 


CHAPTER   V. 

PHYSIOLOGICAL    INTEGRATION    TN    PLANTS. 

§  283.  A  good  deal  has  been  implied  on  this  topic  in  the 
preceding  chapters.  Here,  however,  we  must  for  a  brief 
space  turn  our  attention  immediately  to  it. 

Plants  do  not  display  integration  in  such  distinct  and 
multiplied  ways  as  do  animals.  But  its  advance  may  be 
traced  both  directly  and  indirectly — directly  in  the  increas- 
ing co-ordination  of  actions,  and  indirectly  in  the  effect  of 
this  upon  the  powers  and  habits. 

Let  us  group  the  facts  under  these  heads :  ascending  in 
both  cases  from  the  lower  to  the  higher  types, 

§  284.  The  inferior  Alga,  along  with  little  unlikeness  of 
parts,  show  us  little  mutual  dependence  of  parts.  Having 
surfaces  similarly  circumstanced  everywhere,  much  physio- 
logical  division  of  labour  cannot  arise ;  and  therefore  there 
cannot  be  much  physiological  unity.  Among  the  superior 
Alyte,  however,  the  differentiation  between  the  attached  part 
and  the  free  part  is  accompanied  by  some  integration.  There 
is  evidently  a  certain  transfer  of  materials,  which  is  doubtless 
facilitated  by  the  elongated  forms  of  the  cells  in  the  stem, 
and  probably  leads  to  the  formation  of  dense  tissue  at  the 
places  of  greatest  strain,  in  a  way  akin  to  that  recently  ex- 
plained in  other  cases.  And  where  there  is  this  co-ordina- 
tion of  actions,  the  parts  are  so  far  mutually  dependent  that 
each  dies  if  detached  from  the  other.  That  though  the 


276  PHYSIOLOGICAL    DEVELOPMENT. 

organization  is  so  low  neither  part  can  reproduce  the  other 
and  survive  by  so  doing,  is  probably  due  to  the  circumstance 
that  neither  part  contains  any  considerable  stock  of  untrans- 
formed  protoplasm,  out  of  which  new  tissues  may  be  pro- 
duced. 

Fungi  and  Lichens  present  no  very  significant  advances 
of  integration.  We  will  therefore  pass  at  once  to  the 
Acrogens.  In  those  of  them  which,  either  as  single  fronds 
or  strings  of  fronds,  spread  over  surfaces,  and  which,  rooting 
themselves  as  they  spread,  do  not  need  that  each  part  should 
receive  aid  from  remote  parts,  there  is  no  developed  vascular 
system  serving  to  facilitate  transfer  of  nutriment :  the  parts 
being  little  differentiated  there  is  but  little  integration.  Jiut 
along  with  assumption  of  the  upright  attitude  and  the  ac- 
companying specializations,  producing  vessels  for  distribu- 
ting sap  and  hard  tissue  for  giving  mechanical  support,  there 
arises  a  decided  physiological  division  of  labour ;  rendering 
the  aerial  part  dependent  on  the  imbedded  part  and  the  im- 
bedded part  dependent  on  the  aerial  part.  Here,  indeed,  as 
elsewhere,  these  concomitant  changes  are  but  two  aspects  of 
the  same  change.  Always  the  gain  of  power  to  discharge  a 
special  function  involves  a  loss  of  power  to  perform  other 
functions  ;  and  always,  therefore,  increased  mutual  dependence 
constituting  physiological  integration,  must  keep  pace  with 
that  increased  fitting  of  particular  parts  to  particular  duties 
which  constitutes  physiological  differentiation. 

Making  a  great  advance  among  the  Acrogens,  this  physio- 
logical integration  reaches  its  climax  among  Endogens  and 
Exogens.  In  them  we  see  interdependence  throughout 
masses  that  are  immense.  Along  with  specialized  appli- 
ances for  support  and  transfer,  we  find  an  exchange  of  aid  at 
great  distances.  We  see  roots  giving  the  vast  aerial  growth 
a  hold  tenacious  enough  to  withstand  violent  winds,  and 
supplying  water  enough  even  during  periods  of  drought ;  we 
see  a  stem  and  branches  of  corresponding  strength  for  up- 
holding the  assimilating  organs  under  ordinary  and  extraor- 


PHYSIOLOGICAL    INTEGRATION    IN    PLANTS.  277 

dinary  strains;  and  in  these  assimilating  organs  we  see 
elaborate  appliances  for  yielding  to  the  stem  and  roots  the 
materials  enabling  them  to  fulfil  their  offices.  As  a  con- 
sequence of  which  greater  integration  accompanying  the 
greater  differentiation,  there  is  ability  to  maintain  life  over 
an  immense  period  under  marked  vicissitudes. 

Even  more  conspicuously  exemplified  in  Phaenogams,  is  that 
physiological  integration  which  holds  together  the  functions 
not  of  the  individual  only  but  of  the  species  as  a  whole.  The 
organs  of  reproduction,  both  in  their  relations  to  other  parts 
of  the  individual  bearing  them  and  in  their  relations  to 
corresponding  parts  of  other  individuals,  show  us  a  kind  of 
integration  conducing  to  the  better  preservation  of  the  race  ; 
as  those  already  specified  conduce  to  the  better  preservation  of 
the  individual.  In  the  first  place,  this  greater  co-ordination 
of  functions  just  described,  itself  enables  Phaenogams  to  be- 
queath to  the  germs  they  cast  off,  stores  of  nutriment,  pro- 
tective envelopes,  and  more  or  less  of  organization  :  so  giving 
them  greater  chances  of  rooting  themselves.  In  the  second 
place,  certain  differentiations  among  the  parts  of  fructification, 
the  meaning  of  which  Mr.  Darwin  has  so  admirably  explained, 
give  to  the  individuals  of  the  species  a  kind  of  integration 
that  makes  possible  a  mutual  aid  in  the  production  of 
vigorous  offspring.  And  it  is  interesting  to  observe  how,  in 
that  dimorphism  by  which  in  some  cases  this  mutual  aid  is 
made  more  efficient,  the  greater  degree  of  integration  is 
dependent  on  the  greater  degree  of  differentiation — not  simply 
differentiation  of  the  fructifving  organs  from  other  parts  of  the 
plant  bearing  them,  but  differentiation  of  these  fructifying 
organs  from  the  homologous  organs  of  neighbouring  indi- 
viduals of  the  same  race.  Another  form  of  this 
co-ordination  of  functions  that  conduces  to  the  maintenance  of 
the  species,  may  be  here  named — partly  for  its  intrinsic 
interest.  I  refer  to  the  strange  processes  of  multiplication 
that  occur  in  the  genus  Bryophyllum.  It  is  well  known  that 
the  succulent  leaves  of  B.  calycinum,  borne  on  foot-stalks 


278  PHYSIOLOGICAL    DEVELOPS  KM 

BO  brittle  that  they  are  easily  snapped  by  the  wind,  send 
forth  from  their  edges  when  they  fall  to  the  ground,  buds 
that  root  themselves  and  grow  into  independent  plants.  The 
correlation  here  obviously  furthering  the  preservation  of  the 
race,  is  more  definitely  established  in  another  species  of  the 
genus — B.  proliferum.  This  plant,  shooting  up  to  a  consider- 
able height,  and  having  a  stem  containing  but  little  woody 
fibre,  habitually  breaks  near  the  bottom  while  still  in  flower ; 
and  is  thus  generally  prevented  from  ripening  its  seeds.  The 
multiplication  is,  however,  secured  in  another  way.  Before 
the  stem  is  broken  young  plants  have  budded  out  from  the 
pedicels  of  the  flowers,  and  have  grown  to  considerable  lengths ; 
and  on  the  fall  of  the  parent  they  forthwith  commence  their 
separate  lives.  Here  natural  selection  has  established  a 
remarkable  kind  of  co-ordination  between  a  special  habit  of 
growth  and  decay,  and  a  special  habit  of  proliferation. 

§  285.  The  advance  of  physiological  integration  among 
plants  as  we  ascend  to  the  higher  types,  is  implied  by  their 
greater  constancy  of  structure,  as  well  as  by  the  stricter  limi- 
tation of  their  habitats  and  modes  of  life.  "  Complexity  of 
structure  is  generally  accompanied  with  a  greater  tendency 
to  permanence  in  form,"  says  Dr.  Hooker ;  or,  conversely, 
"  the  least  complex  are  also  the  most  variable."  This  is  the 
second  aspect  under  which  we  have  to  contemplate  the  facts. 

The  differences  between  the  simpler  Alga  and  Fungi,  and 
between  them  and  the  Lichens,  are  so  feebly  marked  that 
botanists  have  been  unable  to  frame  satisfactory  definitions 
of  these  classes.  "  Linnaeus,  for  instance,  and  Jussieu,  con- 
sideied  Lichens  as  forming  a  part  of  Algce,  in  which  they 
are  followed  by  Fries."  Mr.  Berkeley,  however,  quoting  the 
admission  of  Fries  "  that  there  is  no  certain  distinction  be- 
tween Lichens  and  Fungi,  except  the  presence  in  the  former 
of  green  globules,  resembling  grains  of  chlorophyll,"  him- 
self prefers  to  unite  Fungi  and  Lichens  under  the  general 
head  of  Mycetales.  This  structural  indefiniteness  is  accom- 


PHYSIOLOGICAL    INTEGRATION    IN    PLANTS.  270 

panied  by  functional  indefiniteness.  Though,  considered 
collectively,  these  Thallogens  form  "  three  very  natural 
groups,  according  as  they  inhabit  the  water,  the  earth,  or 
the  air;"  yet  if,  instead  of  their  higher  members  we  look  at 
their  lower  members,  we  find  these  distinctions  of  habitat  very 
undecided.  Algee,  which  are  mostly  aquatic,  include  many 
small  forms  that  frequent  the  damp  places  preferred  by 
Lichens  and  Fungi.  Among  Lichens,  as  among  Fungi,  there 
are  kinds  that  lead  submerged  lives  like  the  Alga.  While 
terrestrial  Lichens  and  Fungi  compete  for  the  same  places,  as 
well  as  simulate  one  another's  modes  of  growth.  Besides 
this  indistinctness  of  the  classes,  there  is  great  variability  in 
the  shapes  and  modes  of  life  of  their  species — a  variability 
so  great  that  what  were  at  first  taken  to  be  different  species 
or  different  genera,  or  even  different  orders,  have  proved  to 
be  merely  varieties  of  one  species.  So  inconstant  in  struc- 
ture are  the  Alga  that  Schleiden  quotes  with  approval  the 
opinion  of  Kutzing,  that  "  there  are  no  species  but  merely 
forms  of  Algce."  In  all  which  groups  of  facts  we  see  that 
these  lowest  types  of  plants,  little  differentiated,  are  also  but 
little  integrated. 

Acrogens  present  a  parallel  relation  between  the  small 
specialization  of  functions  which  constitutes  physiological 
differentiation,  and  the  small  combination  of  functions  which 
constitutes  physiological  integration.  "  Mosses,"  says  Mr. 
Berkelej',  "are  no  less  variable  than  other  cryptogams, 
and  are  therefore  frequently  very  difficult  to  distinguish. 
Not  only  will  the  same  species  exhibit  great  diversity 
in  the  size,  mode  of  branching,  form  and  nervation  of  the 
leaves,  but  the  characters  of  even  the  peristome  itself  are 
not  constant."  And  concerning  the  classification  of  the 
remaining  group,  Filicales,  he  says: — "Not  only  is  there 
great  difficulty  in  arranging  ferns  satisfactorily,  but  it  is 
even  more  difficult  to  determine  the  limits  of  species." 

After  this  vagueness  of  separation  as  well  as  inconstancy 
of  structure  and  habit  among  the  lower  plants,  the  stability 


280  PHYSIOLOGICAL    DEVELOPMENT. 

of  structure  and  habit  and  divisibility  of  groups  among 
the  higher  plants,  appear  relativel}1-  marked.  Though 
Phaenogams  are  much  more  variable  than  most  botanists  have 
until  recently  allowed,  yet  the  definitions  of  species  and 
genera  may  be  made  with  far  greater  precision  and  are 
far  less  capable  of  change  than  among  Cryptogams. 
And  this  comparative  fixity  of  type,  implying,  as  it 
does,  a  closer  combination  of  the  component  functions,  we 
see  to  be  the  accompaniment  of  the  greater  differentiation  of 
those  functions  and  of  the  structures  performing  them.  That 
these  characters  are  correlatives  is  further  shown  by  the 
fact  that  the  higher  plants  are  more  restricted  in  their 
habitats  than  the  lower  plants,  both  in  space  and  time.  "  The 
much  narrower  delimitation  in  area  of  animals  than  plants," 
says  Dr.  Hooker,  "and  greater  restriction  of  Faunas  than 
Floras,  should  lead  us  to  anticipate  that  plant  types  are, 
geologically  speaking,  more  ancient  and  permanent  than  the 
higher  animal  types  are,  and  so  I  believe  them  to  be,  and  I 
would  extend  the  doctrine  even  to  plants  of  highly  complex 
structure."  "  Those  classes  and  orders  which  are  the  least 
complex  in  organization  are  the  most  widely  distributed." 

§  286.  Thus  that  which  the  general  doctrine  of  evolution 
leads  us  to  anticipate,  we  find  implied  by  the  facts.  The 
physiological  division  of  labour  among  parts,  can  go  on  only 
in  proportion  to  the  mutual  dependence  of  parts ;  and  the 
mutual  dependence  of  parts  can  progress  only  as  fast  as  there 
arise  structures  by  which  the  parts  are  efficiently  combined, 
and  the  mutual  utilization  of  their  actions  made  easy. 

To  say  definitely  by  what  process  is  brought  about  this 
co-ordination  of  functions  which  accompanies  their  specializa- 
tion, is  hardly  practicable.  Direct  and  indirect  equilibration 
doubtless  co-operate  in  establishing  it.  We  may  see,  for 
example,  that  every  increase  of  fitness  for  function  produced 
in  the  aerial  part  of  a  plant  by  light,  as  well  as  every  increase 
of  fitness  for  function  produced  in  its  imbedded  part  by  the 


PHYSIOLOGICAL    INTEGRATION    IN    PLANTS.  281 

direct  action  of  the  moist  earth,  must  conduce  to  an  increased 
current  of  the  liquid  evaporated  from  the  one  and  supplied 
by  the  other — must  serve,  therefore,  to  aid  the  formation  of 
8ap-channels  in  the  ways  already  described;  that  is — must 
serve  to  develop  the  structures  through  which  mutual  aid  of 
the  parts  is  given :  the  additional  differentiation  tends  imme- 
diately to  bring  about  the  additional  integration.  Con- 
trariwise, it  is  obvious  that  an  interdependence  such  as  we 
see  between  the  secretion  of  honey  and  the  fertilization  of 
germs,  or  between  the  deposit  of  albumen  in  the  cotyledons 
of  an  embryo-plant  and  the  subsequent  striking  root,  is  a  kind 
of  integration  in  the  actions  of  the  individual  or  of  the 
species,  which  no  differentiation  has  a  direct  tendency  to 
initiate.  Hence  we  must  regard  the  total  results  as  due  to  a 
plexus  of  influences  acting  simultaneously  on  the  individual 
and  on  the  species :  some  chiefly  affecting  the  one  and  some 
chiefly  affecting  the  other. 


CHAPTER  VT. 

DIFFERENTIATIONS   BETWEEN  THE   OUTER  AND   INNER 
TISSUES  OF  ANIMALS. 

§  287.  What  was  said  respecting  the  primary  physiological 
differentiation  in  plants,  applies  with  little  beyond  change  of 
terras  to  animals.  Among  Protozoa,  as  among  Protophyta, 
the  first  definite  contrast  of  parts  that  arises  is  that  between 
outside  and  inside.  The  speck  of  jelly  or  sarcode  which  appears 
to  constitute  the  simplest  animal,  proves,  on  closer  examina- 
tion, to  be  a  mass  of  substance  containing  a  nucleus — a 
periplast  in  the  midst  of  which,  there  is  a  minute  endoplast, 
consisting  of  a  spherical  membrane  and  its  contents. 

This  parallel,  only  just  traceable  among  these  Rhizopods, 
which  are  perpetually  changing  the  distribution  of  (heir  outer 
substance,  becomes  at  once  marked  in  those  higher  Protozoa 
which  have  fixed  shapes,  and  maintain  constant  relations 
between  their  surfaces  and  their  environments.  Indeed  the 
Rhizopods  themselve?,  on  passing  into  a  state  of  quiescence 
in  which  the  relations  of  outer  and  inner  parts  are  fixed, 
become  encysted :  there  is  formed  a  hardened  outer  coat 
different  from  the  matter  which  it  contains.  And  what  is 
here  a  temporary  character  answering  to  a  temporary 
definiteness  of  conditions,  is  in  the  Infusoria  a  constant 
character,  answering  definite  conditions  that  are  constant. 
Each  of  these  minute  creatures,  though  not  coated  by  a  dis- 
tinct membrane,  has  the  outer  layer  or  its  sarcode  indurated : 
the  indurated  substance  being  not  separable  from  the  sub- 
stance inclosed,  but  passing  into  it  insensibly. 


THE    OUTER    AND    INNER    TISSUES    OF    ANIMALS.  28o 

§  288.  The  early  establishment  of  this  primary  contrast  of 
tissues  answering  to  this  primary  contrast  of  conditions,  is  no 
less  conspicuous  in  aggregates  of  the  second  order.  The 
ieebly-integrated  units  of  a  Sponge,  with  individualities  so 
little  merged  in  that  of  the  whole  they  form  that  most  of 
them  still  retain  their  separate  activities,  nevertheless  show 
us,  in  the  unlikeness  that  arises  between  the  outermost  layer 
and  the  contained  mass,  the  effect  of  converse  with  unlike 
conditions.  This  outermost  layer  is  composed  of  units  some- 
what flattened  and  united  into  u  continuous  membrane — a 
kind  of  rudimentary  skin. 

Secondary  aggregates  in  which  the  lives  of  the  units  arc 
more  subordinate  to  the  life  of  the  whole,  cany  this  dis- 
tinction further.  The  leading  physiological  trait  of  every 
coelenterate  animal  is  the  divisibility  of  its  substance  into 
endoderm  and  ectoderm — the  part  next  the  food  and  the  part 
next  the  environment.  Fig.  147,  rudely  representing  a  por- 
tion of  the  body-wall  of  a  Hydra  seen  in  section,  gives  some 
idea  of  this  fundamental  differentiation.  The  creature  con- 
sists of  a  simple  sac,  the  cavity  of  which  is  in  direct  commu- 
nication with  the  surrounding  water  ;  and  hence  there  is  but 
little  unlikeness  between  the  outer  and  inner  layers  :  indeed 
they  are  said  to  be  capable  of  exchanging  their  functions. 
The  essential  contrast  is  that  between  the  parts  in  contact 
with  foreign  substances  and  the  parts  sheltered  from  them — 
between  the  developed  surfaces  of  the  endoderm  and  ectoderm, 
and  that  intermediate  stratum  of  nucleated  sarcode  from 
which  the  two  grow  in  opposite  directions. 

Between  this  case  and  the  case  of  the  Sponge,  we  may 
readily  trace  the  connexion.  Suppose  a  mass  of  Amoeba-form 
units,  the  outermost  of  which  are  united  into  a  layer  analogous 
to  that  by  which  a  living  Sponge  is  covered,  to  be  represented 
by  a  lump  of  plastic  clay  ;  and  for  convenience  of  identifica- 
tion, suppose  the  surface  of  the  clay  to  be  coated  by  an 
extensible  film,  say  of  caoutchouc.  Let  this  clay,  so  coated, 
be  moulded  into  the  shape  of  a  cup ;  the  cup  be  gradually 


284  PHYSIOLOGICAL   DEVELOPMENT. 

deepened  until  it  becomes  jar-shaped  ;  and  finally,  by  narrow- 
ing its  neck,  vase-shaped.  And  conceive  the  stretched  film 
to  continue  everywhere  covering  the  surface  during  these 
changes  of  form.  What  will  finally  be  the  relations  of  the 
parts  to  one  another  ?  The  caoutchouc  will  line  the  inside  of 
the  vase  as  well  as  coat  its  outside.  The  vase  will  consist  of 
a  stratum  of  the  clay  included  between  the  two  India-rubber 
surfaces.  -We  shall  have  a  distribution  of  layers  answering 
completely  to  the  distribution  of  tissues  in  the  Hydra.  Now 
if  we  imagine  that  this  artificial  layer  which  has  covered  the 
clay  during  its  changes  of  form,  is  produced  by  transforma- 
tion of  the  clay,  we  shall  see  that  when  the  mass  is  changed 
into  the  vase-shape,  the  surfaces  that  have  become  outer  and 
inner  will  develop  in  opposite  directions  from  the  substance 
lying  between  them ;  just  as  do  the  Hydra's  ectoderm  and 
endoderm.  And  if,  once  more,  we  conceive  these  outer  and 
inner  surfaces  so  resulting,  to  be  affected  by  conditions  some- 
what unlike — the  one  by  matters  placed  in  the  jar,  and  the 
other  by  the  medium  surrounding  the  jar — we  shall  have,  in 
the  slight  difference  produced  between  them,  a  difference 
corresponding  to  that  between  the  surfaces  of  the  Hydra's 
stomach  and  skin. 

Besides  being  able  thus  to  understand  how  an  aggregate 
of  Amoeba-form,  units,  originally  coated  by  a  single  layer, 
may  pass  into  an  aggregate  composed  of  a  double  layer ;  we 
may  also  understand  under  what  influences  the  transition 
takes  place.  If  the  habit  which  some  of  the  primary  aggre- 
gates have,  of  wrapping  themselves  round  masses  of  nutri- 
ment, is  followed  by  a  secondary  aggregate,  there  will 
naturally  arise  just  that  re- differentiation  which  the  Hydra 
shows  us. 

§  289.  These  duplicated  surfaces  which  we  see  in  every  simple 
ccelenterate  animal,  are  re-duplicated  in  all  animals  of  higher 
classes — the  more  developed  Ccelentcrata  themselves  showing 
us  the  transition.  "  Compared  with  the  Hydroid  Polypes/ 


THE    OUTER   AND   INNER   TISSUES   OF    ANIMALS.  285 

says  Prof.  Huxley,  "  the  higher  forms  are  double  animals, 
and  a  section  of  their  bodies  is,  morphologically  speaking,  like 
a  section  of  two  Hydrce,  one  contained  within  the  other." 
The  relations  of  the  parts  may  be  illustrated  thus : — Cut  off 
the  finger  of  a  leather  glove  that  has  a  lining ;  and  let  the 
leather  and  the  lining  represent  the  ectoderm  and  endoderm 
of  a  Hydra.  Thrust  the  point  of  the  glove-finger  back  into 
the  cavity,  until  the  introverted  portion  comes  out  beyond 
the  open  end.  Cut  off  the  projecting  apex  of  the  introverted 
portion  level  with  the  edges  of  the  open  end  ;  and  then  unite 
the  edges  of  the  introverted  portion  and  the  outer  portion. 
The  arrangement  of  structures  will  then  typify  that  which  is 
common  to  all  animals  except  the  Protozoa  and  the  lower 
Ccelenlerata  :  the  introverted  part  representing  the  alimentary 
canal ;  the  outer  part  representing  the  body- wall ;  and  the 
closed  cavity  between  the  two  representing  the  peri- visceral 
sac.  This,  however,  is  not  the  whole  parallelism.  If  in  the 
glove-finger,  representing  in  its  original  form  the  Hydra,  we 
suppose  the  leather  standing  for  the  ectoderm  to  be  growing 
outwards,  and  the  lining  standing  for  the  eudoderm  to  be 
growing  inwards,  then  if  in  the  part  that  is  introverted  the 
same  relations  of  growth  are  maintained,  it  is  manifest  that 
of  its  two  layers  the  one  which  was  outermost  and  is  now 
innermost,  will  grow  towards  the  open  cavity  which  stands 
for  the  alimentary  canal,  while  the  other  layer  will  grow 
towards  the  closed  cavity  standing  for  the  peri- visceral  sac. 
And  these  are  the  directions  of  growth  actually  found  in  the 
parts  thus  symbolized. 

This  simile  must  not  have  more  meaning  given  to  it  than 
is  intended.  Though  there  is  reason  for  suspecting  that  a 
re-duplication  has  taken  place  in  the  course  of  evolution,  and 
that  the  peri-visceral  sac  which  distinguishes  all  the  higher 
classes  of  animals  from  the  lower,  has  been  formed  by  it;  yet 
the  method  of  re-duplication  cannot  have  been  anything  like 
that  described  ;  and  has  probably  been  so  different  a  one  as 
to  negative  the  implied  homologies  of  the  layers.  The  ill  us- 


280  PHYSIOLOGICAL   DEVELOPMENT. 

tration  is  here  used  merely  to  convey,  in  a  way  easy  to 
follow,  an  idea  of  the  relations  between  outer  and  inner 
tissues,  as  they  exist  in  the  more  complex  animals.  The  two 
facts  which  we  have  to  note  are  these  : — First  that,  as  Prof. 
Huxley  points  out  in  his  essay  on  "  Tegumentary  Organs," 
the  course  of  differentiation  in  the  body-  wall  of  the  Hydra,  is 
paralleled  by  the  course  of  differentiation  in  the  skin  of  every 
more  complex  animal  up  to  the  highest  mammal.  Between 
the  epidermis  and  the  derma  there  is  a  layer  of  indifferent 
tissue  corresponding  to  the  layer  that  lies  between  the  endo- 
derm  and  ectoderm  of  the  Hydra ;  and  from  this  layer,  as 
from  its  homologue,  the  differentiations  proceed  in  opposite 
directions.  Though  the  resulting  two  layers,  exposed  to 
more  unlike  conditions  than  those  of  the  Hydra,  are  more 
unlike  one  another,  yet  we  see  in  them  essentially  the  same 
course  of  metamorphosis  and  the  same  subordination  of  it  to 
the  relations  of  outside  and  inside.  In  the  second  place,  we 
have  to  note  that  the  wall  of  the  alimentary  canal,  though  it 
is  in  one  sense  internal  by  contrast  with  the  skin  as  external, 
and  is  correspondingly  differentiated  from  the  skin,  is  in 
another  sense  like  the  skin,  in  having  one  surface  in  contact 
with  foreign  substances  (presented  as  food)  and  the  other 
surface  in  contact  with  the  living  substance  of  the  body  ;  and 
that  consequently  it  undergoes,  like  the  skin,  a  differentia- 
tion into  two  layers,  one  growing  towards  the  relatively 
external  or  food-containing  cavity,  and  the  other  towards  the 
rigorously  internal  cavity — the  closed  peri- visceral  sac. 

§  290.  Whether  direct  equilibration  or  indirect  equilibra- 
tion has  had  the  greater  share  in  producing  this  universally- 
present  contrast  between  the  inner  and  outer  tissues  of 
animals,  must  be  left  undecided.  The  two  causes  have  all 
along  co-operated — modification  of  the  individual  accumu- 
lated by  inheritance  predominating  in  some  cases,  and  in 
other  cases  modification  of  the  race  bv  survival  of  the  inci- 
dentally fittest.  On  the  one  hand,  the  action  of  the  medium 


THE   OUTER   AND    IN.\ER    TISSUES   OF   ANIMALS.  287 

on  the  organism  cannot  fail  to  change  its  surface  more 
than  its  centre,  and  so  differentiate  the  two  ;  while  on  the 
other  hand,  the  surfaces  of  organisms  inhabiting  the  same 
medium  display  extreme  unlikenesses  which  cannot  be  due  to 
the  immediate  actions  of  their  medium.  Let  us  dwell  a 
moment  on  the  antithesis. 

"VVe  have  abundant  evidence  that  animal  protoplasm  is 
rapidly  modified  by  light,  heat,  air,  water,  and  the  salts 
contained  in  water — coagulated,  turned  from  soluble  into  in- 
soluble, partially  changed  into  isomeric  compounds,  or  other- 
wise chemically  altered.  Immediate  metamorphoses  of  this 
kind  are  often  obviously  produce!  in  ova  by  changes  of  their 
media.  At  the  outset,  therefore,  before  yet  there  existed 
any  such  differentiation  as  that  which  now  usually  arises  by 
inheritance,  these  environing  agencies  must  have  tended  to 
originate  a  protective  envelope.  For  a  modification  produced 
by  them  on  the  superficial  part  of  the  protoplasm,  must 
either  have  been  a  decomposition  or  else  the  formation  of  a 
compound  that  remained  stable  under  their  subsequent  action. 
There  would  be  generated  an  outer  layer  of  substance  that 
was  so  molecularly  immobile  as  to  be  incapable  of  further 
metamorphoses,  while  it  would  shield  the  contained  proto- 
plasm from  that  too  great  action  of  external  forces  which,  by 
rapidly  changing  the  unstable  equilibrium  of  its  molecules 
into  a  relatively  stable  equilibrium,  would  arrest  development. 
Evidently  organic  evolution,  whether  individual  or  general, 
must  always  and  everywhere  have  been  subordinate  to  these 
physical  necessities.  Though  natural  selection,  beginning 
with  minute  portions  of  protoplasm,  must  all  along  have 
tended  to  establish  a  molecular  composition  apt  to  undergo 
this  differentiation  of  surface  from  centre  to  the  most  favour- 
able extent ;  yet  it  must  all  along  have  done  so  while  con- 
trolled by  this  process  of  direct  equilibration. 

Contrariwise,  the  many  and  great  uiilikenesses  among  the 
dermal  structures  of  creatures  inhabiting  the  same  element, 
cannot  be  ascribed  to  any  such  cause.  The  contrasts  between 
VOL.  II.  13 


288 


PHYSIOLOGICAL    DE  VELOPM ENT. 


naked  and  shelled  Gastropods,  between  marine  Worms  and 
Crustaceans,  between  soft-skinned  Fish  and  Fish  in  armour  like 
the  Ptericlhys,  must  have  beon  produced  entirely  by  natural 
selection.  Environing  forces  are,  as  before,  the  ultimate 
causes ;  but  the  forces  are  now  not  so  much  those  exercised 
by  the  medium  as  those  exercised  by  the  other  inhabitants  of 
the  medium  ;  and  they  do  not  act  by  modifying  the  surface 
of  the  individual,  but  by  killing  off  individuals  whose  surfaces 
are  least  fitted  to  the  requirements :  thus  slowly  affecting  the 
species.  The  dermal  skeleton  bristling  with  spines,  which 
protects  the  Diodon  or  the  Cydictkys  from  enemies  it  could 
not  escape,  still  comes  within  the  general  formula  of  an  outer 
tissue  differentiated  from  inner  tissues  by  the  outer  actions  to 
which  the  creature  is  exposed — the  differentiation  having 
gone  on  until  there  is  equilibrium  between  the  destructive 
forces  to  be  met  and  the  protective  forces  which  meet  them. 

If  we  venture  to  apportion  the  respective  shares  which 
mediate  and  immediate  actions  have  had  in  differentiating 
outer  from  inner  tissues,  we  shall  probably  not  be  far  wrong 
in  ascribing  that  part  of  the  process  which  is  alike  in  all 
animals,  mainly  to  the  direct  actions  of  their  media ;  while 
we  ascribe  the  multitudinous  unlikenesses  of  the  process  in 
various  animals,  partly  to  the  indirect  actions  of  the  media, 
and  partly  to  the  indirect  actions  of  other  animals  by  which 
the  media  are  inhabited.  That  is  to  say,  while  assigning  the 
specialities  of  the  differentiations  to  the  specialities  of  con- 
verse with  the  agencies  in  the  environment,  most  of  them 
organic,  we  may  assign  to  the  constant  and  universal  con- 
verse with  its  inorganic  agencies,  that  universal  characteristic 
of  tegumentary  structures — their  development  into  a  double 
layer  separated  by  undifferentiated  substance,  from  which  the 
outermost  grows  outwardly  and  the  innermost  grows  in- 
wardly. 

Here  let  me  add  a  piece  of  evidence  which  strengthens 
very  greatly  the  general  nrgument,  at  the  same  time  that  it 
justifies  this  apportionment.  When  ulceration  has  gone  deep 


THE    OUTEA   AND    INNER   TISSUES   OF   ANIMALS.  289 

enough  to  destroy  the  tegumentary  structures,  these  are  never 
reproduced.  The  puckered  surface  formed  where  an  ulcer 
heals,  consists  of  modified  connective  tissue,  which,  as  the 
healing  goes  on,  spreads  inwards  from  the  edges  of  the  ulcer 
— some  of  it,  perhaps,  growing  from  the  portions  of  connective 
tissue  that  dip  down  between  the  muscular  bundles.  This 
connective  tissue,  mark,  out  of  which  is  thus  constituted  the 
make-shift  skin,  is  normally  covered  by  both  the  epidermis 
and  that  stratum  of  indifferent  tissue  from  which  the  growth 
proceeds  in  opposite  directions — is  the  inner  layer  that  grows 
inwardly.  What  has  happened  to  it  ?  It  has  now  become 
the  outermost  layer.  And  how  does  it  comport  itself  under 
its  new  conditions  ?  It  produces  a  layer  that  plays  the  part 
of  epidermis  and  grows  outwardly.  For  since  the  surface, 
subject  to  Motion  and  exfoliation,  has  to  be  continually 
renewed,  there  must  be  a  continual  reproduction  of  a  super- 
ficial layer  from  a  layer  beneath.  That  is  to  say,  the  con- 
tact of  this  deep-seated  tissue  with  outer  agencies,  produces 
in  it  some  approach  towards  that  composition  which  we  find 
universally  characterizes  outer-tissue — a  protomorphio  layer, 
which  differentiates  in  opposite  directions.  But  while  we  see 
under  this  exposure  to  the  conditions  common  to  all  integu- 
ment, a  tendency  to  assume  the  structure  common  to  all 
integument,  we  see  no  tendency  to  assume  any  of  the 
specialities  of  tegumentary  structure :  no  rudiments  of  glands 
or  hair  sacs  make  their  appearance. 

This  apportionment  we  shall  see  the  more  reason  to  accept 
as  approximately  expressing  the  truth,  on  remembering  that 
the  mode  of  differentiation  of  outer  from  inner  tissues  which 
is  common  to  all  animals  is  common  to  all  plants ;  and 
on  observing,  further,  that  the  more  special  interpretation 
suggested  as  not  improbable  in  the  case  of  plants,  is  not 
improbable  in  the  case  of  animals.  For  as  it  was  argued 
that  in  plants  the  forces  evolved  from  within  the  organism, 
and  the  forces  falling  on  it  from  without,  must  have  some 
place  between  centre  and  surface  at  Avhich  they  balance ;  and 


290  PHYSIOLOGICAL    DEVELOPMENT. 

that  at  this  place  will  lie  the  unstable  protoplasm  that 
develops  outwardly  into  a  substance  which  is  stable  in  face 
of  outer  forces,  and  inwardly  into  a  substance  which  is  stable 
in  face  of  inner  forces ;  so  in  animals,  we  may  regard  this 
universally-present  layer  whence  epidermis  grows  outwardly 
and  connective  tissue  inwardly,  as  similarly  the  place  of 
equilibrium  between  these  antagonist  forces.  And  for  this 
d  priori  interpretation  we  may  indeed,  among  animals,  find 
d  posteriori  warrant.  We  have  but  to  increase  the  mechanical 
action  or  chemical  irritation  at  some  part  of  an  animal's 
surface,  to  make  this  plane  of  indifferent  tissue  retreat  in- 
wardly ;  for  to  say  that  the  epidermis  becomes  thicker,  is,  in 
mechanical  terms,  to  say  that  the  place  of  equilibrium  between 
outer  and  inner  forces  is  further  from  the  surface. 


CHAPTER  VII. 

DIFFERENTIATIONS  AMONG  THE  OUTER  TISSUES  OF  ANIMALS. 

§  291.  The  outer  tissues  of  animals,  originally  homo- 
geneous over  their  whole  surfaces,  pass  into  a  heterogeneity 
which  fits  their  respective  parts  to  their  respective  conditions. 
So  numerous  and  varied  are  the  implied  differentiations,  that 
it  is  impracticable  here  to  deal  with  them  all  even  in  outline. 
To  trace  them  up  through  classes  of  animals  of  increasing 
degrees  of  aggregation,  would  carry  us  into  undue  detail. 

Did  space  permit,  it  would  be  possible  to  point  out  among 
the  Protozoa,  various  cases  analogous  to  that  of  the  Arcella; 
which  may  be  described  as  like  a  microscopic  Limpet,  having 
a  sarcode  body  of  which  the  upper  surface  has  become  horny, 
while  the  lower  surface  with  its  protruding  pseudopodia, 
retains  the  primitive  jelly-like  character.  That  differentia- 
tions of  this  kind  have  been  gradually  established  among 
these  minute  creatures  through  the  unlike  relations  of  their 
parts  to  the  environment,  is  an  inference  supported  by  cases 
like  that  of  Pamphagus — an  intermediate  form  which  is  like 
the  Amoeba  in  having  no  carapace,  but  "  agrees  with  Arcella 
and  Difflugia  in  having  the  pseudopodia  protrusible  from  one 
extremity  only  of  the  body." 

Many  parallel  specializations  of  surface  among  aggregates 
of  the  second  order  might  be  instanced  from  the  Coelenterata. 
In  the  Hydra,  the  ectoderm  presents  over  its  whole  area  no 
conspicuous  unlikenesses ;  but  there  usually  exist  in  the 
hydroid  polypes  of  superior  types,  decided  contrasts  between 


292  PHYSIOLOGICAL    DEVELOPMENT. 

the  higher  and  lower  parts.  While  the  higher  parts  retain 
their  original  characters,  the  lower  parts  excrete  hard  outer 
layers  yielding  support  and  protection.  Various  stages  of 
the  difierentiation  might  be  followed.  "In  Hydractima" 
says  Prof.  Green,  this  horny  layer  "  becomes  elevated  at 
intervals  to  form  numerous  rough  processes  or  spines,  while 
over  the  general  surface  of  the  ectoderm  its  presence  is 
almost  imperceptible."  In  other  types,  as  in  Cordylophora, 
it  spreads  part  way  up  the  animal's  sides,  ending  indefinitely. 
In  Bimeria  it  "  extends  itself  so  as  to  enclose  the  entire  body 
of  each  polypite,  leaving  bare  only  the  mouth  and  tips  of  the 
tentacles."  While  in  Campanularia  it  has  become  a  partially- 
detached  outer  cell,  into  which  the  creature  can  retract  its 
exposed  parts. 

But  it  is  as  needless  as  it  would  be  wearisome  to  trace 
through  the  several  sub-kingdoms  the  rise  of  these  multiform 
contrasts,  with  the  view  of  seeking  interpretations  of  them. 
It  will  suffice  if  we  take  a  few  groups  of  the  illustrations 
furnished  by  the  higher  animals. 

§  292.  We  may  begin  with  those  modifications  of  surface 
which  subserve  respiration.  Though  we  ordinarily  think  of 
respiration  as  the  quite  special  function  of  a  quite  special 
organ,  yet  originally  it  is  not  so.  Little- developed  animals 
part  with  their  carbonic  acid  and  absorb  oxygen,  through  the 
general  surface  of  the  body.  Even  in  the  lower  types  of  the 
higher  classes,  the  general  surface  of  the  body  aids  largely  in 
aerating  the  blood  ;  and  the  parts  that  discharge  the  greater 
part  of  this  function  are  substantially  nothing  more  than 
slightly  altered  and  extended  portions  of  the  skin. 

Such  differentiations,  marked  in  various  degrees,  are  to  be 
seen  among  Mollnsca.  In  the  Ptcropoda  the  only  modification 
which  appears  to  facilitate  respiration,  is  the  minute  vascularity 
of  one  part  of  the  skin.  In  other  types  the  specialized  parts 
facilitating  the  exchange  of  gases,  are  those  simple  but 
numerous  expansions  of  surface  constituting  the  papilla! ; 


TLUi    OUTER   TISSUES    OF    ANIMAIJS.  293 

which,  iu  the  Eclis  and  kinds  allied  to  it,  are  distributed  in 
rows  or  clusters  all  along  the  back.  Instead  of  these,  the 
Doris  has  appendages  developed  into  elaborately-branched 
forms — small  trees  of  blood-vessels  covered  by  slightly- 
changed  dermal  tissues.  And  these  arborescent  branchiae  are 
gathered  together  into  a  single  cluster.  Thus  there  is 
evidence  that  large  external  respiratory  organs  have  arisen 
by  degrees  from  simple  skin  :  as,  indeed,  they  do  arise  during 
the  development  of  each  individual  having  them.  Just  as 
gradually  as  in  the  embryo  the  simple  bud  on  the  integu- 
ment, with  its  contained  vascular  loop,  passes  by  secondary 
buddings  into  a  tree-like  growth  penetrated  everywhere  by 
dividing  and  sub-dividing  blood-vessels ;  so  gradually  has 
there  probably  proceeded  the  differentiation  which  has  turned 
part  of  the  outer  surface  into  an  organ  for  excreting  carbonic 
acid  and  absorbing  oxygen. 

Certain  inferior  vertebrate  animals  present  us  with  a  like 
metamorphosis  of  tissues.  These  are  the  Amphibia.  The 
branchias  here  developed  from  the  skin  are  covered  with  cel- 
lular epidermis,  not  much  thinner  than  that  covering  the  rest 
of  the  body.  Like  it  they  have  their  surfaces  speckled  with 
pigment-cells ;  and  are  not  even  conspicuous  by  their  extra 
vascularity — where  they  are  temporary  at  least.  They  facili- 
tate the  exchange  of  gases  in  scarcely  any  other  -svay  than  by 
affording  a  larger  area  of  contact  with  the  water,  and  inter- 
posing a  rather  thinner  layer  of  tissue  between  the  water 
and  the  blood-vessels.  Those  very  simple  branchiae  of  the 
larval  Amphibia  that  have  them  but  fo'r  a  short  time, 
graduate  into  the  more  complex  ones  of  those  that  have  them 
for  a  long  time  or  permanently  ;  showing,  as  before,  the  small 
stages  by  which  this  heterogeneity  of  surface  accompanying 
heterogeneity  of  function  may  arise. 

In  what  way  are  such  differentiations  established  ?  Partly, 
no  doubt,  by  natural  selection  ;  but  also  to  some  degree,  I 
think,  by  the  inheritance  of  direct  adaptations.  That  a  por- 
tion of  the  integument  at  which  aeration  is  favoured  by  local 


294  PHYSIOLOGICAL    DEVELOPMENT. 

conditions,  should  thereby  be  led  to  grow  into  a  larger 
surface  of  aeration,  appears  improbable  :  survival  of  those 
individuals  which  happen  to  have  this  portion  of  the  integu- 
ment somewhat  more  developed,  seems  here  the  only  likely 
cause.  Nevertheless  there  is  reason  for  suspecting  that 
respiratory  activity  itself  aids  in  the  development  of  a  re- 
spiratory appendage.  The  reason  is  this.  Exchange  of  liquids 
through  membrane  depends  on  some  difference,  physical  or 
chemical,  between  the  liquids  :  if  they  are  in  all  respects 
'  alike,  and  under  equal  pressures,  no  exchange  will  take  place  ; 
while,  conversely,  if  they  are*  much  unlike  there  will  be  a 
rapid  exchange.  Now  through  the  walls  of  capillaries,  or 
through  the  sides  of  lacunas  not  yet  developed  into  capillaries, 
there  continually  goes  on  an  oozing  both  ways — from  the 
blood  into  the  tissues  and  from  the  tissues  into  the  blood. 
By  this  double  movement  nutrition  and  depuration  are  alike 
made  possible ;  and  it  is  obvious  both  that  in  the  absence  of 
difference  it  would  not  occur,  and  that  nothing  would  be 
gained  if  it  did  occur.  Among  other  differences  continually 
arising  between  the  intra- vascular  liquid  and  the  extra- 
vascular  liquid,  is  that  due  to  their  unlike  charges  of 
oxygen  and  carbonic  acid.  This  difference,  like  other  differ- 
ences, will  cause  exchange — the  rapidity  of  the  exchange 
doubtless  being  greater  where  the  difference  is  greater. 
Hence  if  any  part  of  an  aquatic  animal's  skin  is  nearest  to 
the  place  where  the  blood  has  become  most  highly  carbonized, 
or  if  it  is  so  bathed  with  moving  water  that  the  plasma 
beneath  its  surface  is  more  oxygenated  than  elsewhere,  or 
both  ;  then,  other  things  equal,  this  part  of  the  skin  will  be 
the  seat  of  an  osmotic  movement  greater  than  goes  on  in  the 
rest  of  the  skin.  But  the  exchange  of  oxygen  for  carbonic 
acid,  proceeding  faster  here  than  elsewhere,  will  have  for  its 
accompaniment  a  more  rapid  exudation  of  nutritive  matters. 
The  liquid  passing  out  of  the  blood-vessels  to  be  replaced  by 
the  liquid  passing  into  them,  is  a  liquid  containing  the 
substances  that  build  up  the  surrounding  tissues.  Hence 


THE    OUTER    TISSUES    OF    ANIMALS.  xJl» 

theso  tissues  may  be  expected  to  grow  :  the  area  supplied  by 
the  increased  currents  of  blood  set  up  by  this  exchange,  will 
become  protuberant — will  bud  out ;  and  the  bud  so  formed 
will  give  origin  to  secondary  buds  at  those  parts  of  its  surface 
which,  as  before,  are  most  favourably  circumstanced  for 
carrying  on  the  aeration.  Of  course  this  process  will  be 
checked  where,  though  otherwise  advantageously  placed,  the 
growing  branchiae  would  be  specially  liable  to  damage,  or 
would  be  great  hindrances  to  the  creature's  movements.  But 
bearing  in  mind  that  functionally-produced  adaptation  will 
here,  as  in  other  cases,  be  both  aided  and  controlled  by 
natural  selection,  we  may  ascribe  to  it  an  important  share,  if 
not  a  leading  share,  in  the  differentiation. 

§  293.  Among  the  conspicuous  modifications  by  which  the 
originally-uniform  outer  layer  is  rendered  multiform,  are  the 
protective  structures.  Let  us  look  first  at  the  few  cases  in 
which  the  formation  of  these  is  ascribable  mainly  to  direct 
equilibration. 

Already  reference  has  been  more  than  once  made  to  those 
thickenings  that  occur  where  the  skin  is  exposed  to  unusual 
pressure  and  friction.  Are  these  adaptations  inheritable  ? 
and  may  they,  by  accumulation  through  many  generations, 
produce  permanent  dermal  structures  fitted  to  permanent  or 
frequently-recurring  stress  ?  Taking,  for  instance,  the  cal- 
losities on  the  knuckles  of  the  Gorilla,  which  are  adapted  to 
its  habit  of  partially  supporting  itself  on  its  closed  hands 
when  moving  along  the  ground — shall  we  suppose  that  these 
defensive  thickenings  are  produced  afresh  in  each  individual 
by  the  direct  actions  ;  or  that  they  are  inherited  modifica- 
tions caused  by  such  direct  actions;  or  that  they  are  wholly 
due  to  the  natural  selection  of  spontaneous  variations  ? 
The  last  supposition  does  not  seem  a  probable  one ;  since  it 
implies  that  those  slight  extra  thicknesses  of  skin  on  the 
knuckles,  with  which  we  must  suppose  the  selection  to  have 
commenced,  were  so  advantageous  as  to  cause  survivals  of  the 


290  PHYSIOLOGICAL   DEVELOPMENT. 

individuals  having  them.  That  survivals  so  caused,  if  they 
ever  occurred  at  all,  should  have  occurred  with  the  frequency 
requisite  to  establish  and  increase  the  variation,  is  hardly 
supposable.  And  if  we  reject,  as  also  unlikely,  the  repro- 
duction of  these  callosities  de  novo  in  each  individual, 
there  remains  only  the  inference  that  they  have  arisen 
by  the  transmission  and  accumulation  of  functional  adapta- 
tions. Another  case  which  seems  interpretable 
only  in  an  analogous  way,  is  that  of  the  spurs  that  are 
developed  on  the  wings  of  certain  birds — on  those  of  the 
Chaja  screamer  for  example.  These  are  weapons  of  offence 
and  defence.  It  is  a  familiar  fact  that  many  birds  strike 
with  their  wings,  often  giving  severe  blows  ;  and  in  the 
birds  named,  the  blows  are  made  more  formidable  by  the 
horny,  dagger-shaped  growths  standing  out  from  those  points 
on  the  wings  which  deliver  them.  Are  these  spurs  directly 
or  indirectly  adaptive  ?  To  conclude  that  natural  selection 
of  spontaneous  variations  has  caused  them,  is  to  conclude 
that,  without  any  local  stimulus,  thickenings  of  the  skin 
occurred  symmetrically  on  the  two  wings  at  the  places 
required  ;  that  such  thickenings,  so  localized,  happened  to 
arise  in  birds  given  to  using  their  wings  in  fight ;  and  that 
on  their  first  appearance  the  thickenings  were  decided  enough 
to  give  appreciable  advantages  to  the  individuals  distinguished 
by  them — advantages  in  bearing  the  reactions  of  the  blows  if 
not  in  inflicting  the  blows.  But  to  conclude  this  is,  I  think, 
to  conclude  against  probability.  Contrariwise,  if  we  assume 
that  the  thickening  of  the  epidermis  produced  by  habitual 
rough  usage  is  inheritable,  the  development  of  these  struc- 
tures presents  no  difficulty.  The  points  of  impact  would 
become  indurated  in  wings  used  for  striking  with  unusual 
frequency.  The  callosities  of  surface  thus  generated,  render- 
ing the  parts  less  sensitive,  would  enable  the  bird  in  which 
they  arose  to  give,  without  injury  to  itself,  more  violent  blows 
and  a  greater  number  of  them — so,  in  some  cases,  helping  it 
to  conquer  and  survive.  Among  its  descendants,  inheriting 


THE    OUTER   TISSUES    OF    ANIMALS.  297 

the  modification  and  the  accompanying  habit,  the  thickening 
would  be  further  increased  in  the  same  way — survival  of  the 
fittest  tending  ever  to  accelerate  the  process.  Presently  the 
horny  nodes  so  formed,  hitherto  defensive  only  in  their 
effects,  would,  by  their  prominence,  become  offensive  —  would 
make  the  blows  given  more  hurtful.  And  now  natural 
selection,  aiding  more  actively,  would  mould  the  nodes  into 
spurs  :  the  individuals  in  which  the  nodes  were  most  pointed 
would  be  apt  to  survive  and  propagate  ;  and  the  pointedness 
generation  after  generation  thus  increased,  would  end  in  the 
well- adapted  shape  we  see. 

But  if  in  these  cases  the  differentiations  which  fit  particular 
parts  of  the  outer  tissues  to  bear  rough  usage,  are  caused 
mainly  by  the  direct  balancing  of  external  actions  by  in- 
ternal reactions,  then  we  may  suspect  that  the  like  is  true 
of  other  modifications  that  occur  where  special  strains  and 
abrasions  have  to  be  met.  Possibly  it  is  true  of  sundry  parts 
that  are  formed  of  hardened  epidermis,  such  as  the  nails, 
claws,  hoofs,  and  hollow  horns  of  Mammals  ;  "  all  of  which," 
says  Prof.  Huxley,  "  are  constructed  on  essentially  the  same 
plan,  being  diverticula  of  the  whole  integument,  the  outer 
layer  of  whose  ecderon  has  undergone  horny  metamorphosis." 
Leaving  open,  however,  the  question  what  tegumentary 
structures  are  due  to  direct  equilibration,  furthered  and  con- 
trolled by  indirect  equilibration,  it  is  tolerably  clear  that 
direct  equilibration  has  been  one  of  the  factors. 

How  has  it  produced  its  effects  ?  that  is  to  say — by  what 
physical  processes  do  pressure  and  friction  bring  about  dermal 
hardenings  ?  To  this  inquiry  there  is  an  answer  similar  to 
that  which  was  given  to  the  inquiry  respecting  the  formation 
of  wood.  (§  280-2.)  As  in  plants  we  saw  that  intermittent 
compressions  of  sap-canals  increase  the  exudation  of  sap,  and 
thus  cause  increased  deposits  of  its  contained  substances  in 
the  surrounding  tissues  ;  so  in  animals,  we  have  good  reason 
for  concluding  that  intermittent  compressions  of  the  capil- 
laries increase  the  exudation  of  serum,  and  by  thus  supplying 


298  PHYSIOLOGICAL   DEVELOPMENT. 

extra  nutriment  to  the  structures  adjacent,  lead,  other  things 
equal,  to  thickening  or  induration.  The  data  for  the  con- 
clusion are  these  : — Through  the  walls  of  the  capillaries  the 
liquid  plasma  of  the  blood  continually  oozes.  The  oozing 
is  partly  osmotic  and  parti}'  mechanical — partly  due,  that  is, 
to  the  exchange  of  the  unlike  liquids  that  lie  inside  and  out- 
side the  capillaries,  and  partly  to  the  greater  pressure 
put  upon  the  liquid  inside.  That  this  last  is  one  of 
the  causes  is  proved  by  the  phenomena  of  dropsy — a  disease 
in  which  the  exudation  is  unduly  rapid.  Dropsy  in  the  legs 
gets  worse  during  the  day,  when  by  sitting  and  standing  the 
weight  of  the  blood  to  be  borne  by  the  vessels  of  the  legs  is 
increased ;  and  gets  better  during  the  night,  when  by  the 
recumbent  attitude  these  vessels  are  relieved  from  this 
weight.  Contrariwise,  that  cedematous  swelling  under  the 
eyes  which  is  common  in  the  aged  and  debilitated,  increases 
during  the  night  and  decreases  during  the  day — gravitation 
serving,  when  the  body  is  upright,  to  diminish  the  pressure  of 
the  blood  at  this  part,  and  not  having  this  effect  when  the 
body  is  horizontal.  But  if  the  plasma  is  to  some  extent 
forced  through  the  walls  of  the  capillaries  by  pressure,  then 
not  only  will  the  action  of  the  heart,  aided  at  some  parts  by 
gravity,  further  the  exudation,  but  the  exudation  will  be 
furthered  by  external  pressures  from  time  to  time  falling  on 
the  capillaries.  If  the  capillaries  of  the  skin  be  squeezed 
by  the  thrust  of  some  object  against  the  surface,  part  of  their 
contained  blood  will  be  driven  back  into  the  arteries,  more 
will  be  driven  forwards  into  the  veins,  and  some  will  be  made 
to  exude.  Immediately  they  are  relieved  from  the  pressure 
thev  will  be  refilled  from  the  arteries,  again  to  yield  an  extra 
portion  of  their  contents  to  the  tissues  around  when  again 
squeezed.  Thus  recurrent  thrusts  or  impacts,  acting  on  the 
body  from  without,  aid  in  the  nutrition  of  the  parts  on  which 
they  fall :  producing,  in  some  cases,  a  node  upon  the  subjacent 
bone,  as  on  the  instep  where  a  boot  has  pinched  ;  producing, 
in  other  cases,  growth  of  the  connective  tissue,  as  in  a  bunion  j 


THE    OUTER   TISSUES   OF   ANIMALS.  299 

and  producing,  more  frequently,  thickening  of  the  epidermis.* 
It  is  no  doubt  true  that  the  sensation  which  pressure  causes, 
propagated  to  the  spinal  chord,  and  reflected  thence  through 
the  vaso- motor  nerve  going  to  the  spot,  aids  the  process 
by  exciting  a  wave  of  contraction  along  the  minute  arteries, 
thereby  helping  them  to  refill  the  capillaries  the  instant  the 
pressure  is  taken  off;  and  doubtless,  as  alleged,  the  excessive 
exudation  that  forms  a  blister  when  the  intermittent  com- 
pressions are  violent  and  long-continued,  is  attributable  to 
this  reflex  nervous  action.  But  it  is  clear  that  the  nervous 
action  is  secondary,  and  cannot  of  itself  produce  the  effect ; 
for  in  the  absence  of  intermittent  pressure  no  exudation  takes 
place,  however  acute  and  persistent  the  sensation  may  be. 
Continued  pressure  produces  absorption  instead  of  exudation. 
In  animals  therefore,  as  in  plants,  the  external  mechanical 
actions  to  be  resisted,  are  themselves  directly  instrumental  in 
working  in  the  tissues  they  fall  upon,  the  changes  which  fit 
those  tissues  to  meet  them.  And  it  needs  but  to  contemplate 
the  process  of  thickening  described,  to  see  that  it  will  go  on 
until  the  shield  produced  suffices  to  protect  the  capillaries 
frtfm  excessive  pressures — will  go  on,  that  is,  until  there  is 
equilibrium  between  the  outer  and  inner  forces. 

§  294.  Dermal  structures  of  another  class  are  developed 
mainly,  if  not  wholly,  by  the  actions  of  external  causes 
on  species  rather  than  on  individuals.  These  are  the 

*  An  inquiry  into  the  causes  of  these  differences  of  result,  brings  further 
evidence  to  light.  The  condition  under  which  only  the  hypertrophy  can 
arise,  is  that  the  pressure  intermits  sufficiently  to  allow  the  capillaries  to 
refill  frequently.  The  epidermis  thickens  where  the  pressures  are  habitually 
taken  off  so  completely,  that  the  capillaries  next  the  surface  can  refill,  as  in  the 
hands.  If  we  consider  what  happens  where  the  instep  is  pressed  by  a  tight 
boot,  we  shall  see  that  the  variations  of  pressure  which  occur  in  walking,  do 
not  suffice  to  relieve  the  quite  superficial  vessels  and  allow  them  to  refill  ;  but 
in  consequence  of  the  slight  mobility  and  elasticity  of  the  tissues,  the  vessel.' 
»it  some  distance  beneath  the  surface  are  able  to  refill,  and  hence  the  thicken- 
ing occurs  round  them. 


300  PHYSIOLOGICAL   DEVELOPMENT. 

various    kinds   of    clothing — hairs,    feathers,    quills,    scales, 
scutes. 

Headers  who  are  unfamiliar  with  the  extreme  modifiability 
of  organic  structures,  will  be  startled  by  the  proposition  that 
all  of  these — certainly  all  of  them  but  the  last,  respecting 
which   there  may  be   doubts — are   homologous   parts.     In- 
spection of  a  few  cases  makes  this  seemingly-incredible  pro- 
position not  simply  credible  but  obviously  true.    A  retrograde 
metamorphosis  from  feathers  to  appendages  that  are  almost 
scale-like,  is  well  seen  in  the  coat  of  the  Penguin.     Carry 
the  eye  along  the  surface  of  one  of  these  birds,  and  there  is 
manifest  a  transition  from  the  bird-like  covering  to  the  fish- 
like  covering — a  transition  so  gradual  that  no  place  can  be 
found   where   an   appreciable   break   occurs.      Less  striking 
perhaps,  but  scarcely  less  significant,  are  the  modifications 
through  which  we  pass  from  feathers  to  hairs,  on  the  surfaces 
of  the  Ostrich  and  the  Cassowary.    The  skin  of  the  Porcupine 
shows  us  hairs  and  quills  united  by  a  series  of  intermediate 
structures,  differing  from  one  another  inappreciably.     Even 
more  remarkable  is  the  extension  of  this  alliance  to  certain 
other  dermal  structures.     "It  may  be  taken  as  certain,  I 
think,"    says   Prof.    Huxley,   "  that   the   scales,  plates,  and 
spines  of  all  fishes  are  homologous  organs ;  nor  as  less  so 
that  the  tegumentary  spines  of  the  Plagiostomes  are  homo- 
logous with  their  teeth,  and  thence  with  the  teeth   of  all 
vertebrata.     Again,   it  appears  to  me  indubitable  that  the 
teeth  and  the  hairs  are  homologous  organs." 

The  ultimate  justification  for  classing  these  unlike  parts  as 
divergent  modifications  of  the  same  thing,  is  the  unity  in 
their  modes  of  development.  Besides  a  linking  together  of 
them  by  intermediate  structures,  as  above  indicated,  there  is 
a  linking  together  by  their  common  origin.  To  quote  again 
from  Prof.  Huxley's  essay  on  "Tegumentary  Organs": — 
"  The  Hairs  and  fyjines  of  mammals,  the  Feathers  of  birds, 
and  the  Integumentary  Glands,  agree  in  one  essential  point, 
that  their  development  is  preceded  by  that  of  an  involution 


THE    OUTER   TISSUES   OF    ANIMALS.  3D1 

of  the  ecderon,  within  which  they  are  formed,  and  by  which 
the  former  are,  at  first,  entirely  enclosed."  And  though  the 
scales  of  fishes  and  the  dermal  plates  of  reptiles  present  diffi- 
culties, yet  Prof.  Huxley  concludes  that  the  course  of  their 
development  is  at  first  essentially  the  same.  Some  idea  of  it, 
and  of  the  relations  it  proves  among-  these  structures,  may  be 
given  thus : — -Suppose  a  small  pit  to  be  formed  on  the  pre- 
viously flat  skin ;  and  suppose  that  the  growth  and  casting 
off  of  horny  cells  which  goes  on  over  the  skin  in  general, 
continues  to  go  on  at  the  usual  rate  over  the  depressed  surface 
of  this  pit.  Clearly  the  quantity  of  horny  matter  produced 
within  this  hollow,  will  be  greater  than  that  produced  on  a 
level  portion  of  the  skin  subtending  an  equal  area  of  the 
animal's  outside.  Suppose  such  a  pit  to  be  deepened 
until  it  becomes  a  small  sac.  If  the  exfoliation  goes  on  as 
before,  the  result  will  be  that  the  horny  matter,  expelled,  as 
it  must  be,  through  the  mouth  of  the  sac,  which  now  bears 
a  small  proportion  to  the  internal  surface  of  the  sac,  will  be 
large  in  quantity  compared  with  that  exfoliated  from  a 
portion  of  the  skin  equal  in  area  to  the  mouth  of  the  sac : 
there  will  be  a  conspicuous  thrusting  forth  of  horny  matter. 
Suppose  once  more  that  the  sac,  instead  of  remaining  simple, 
has  its  bottom  pushed  up  into  its  interior,  lvke  the  bottom  of 
a  beer- bottle — the  introversion  being  carried  so  far  that  the 
introverted  part  reaches  nearly  to  the  external  opening,  and 
leaves  scarcely  any  space  between  the  introverted  part  and 
the  walls  of  the  sac.  It  is  easy  to  see  that  the  exfoliation 
continuing  from  the  surface  of  the  introverted  part  as  well  as 
from  the  inside  of  the  sac  generally,  the  horny  matter  cast 
off  will  form  a  double  layer ;  and  svill  come  out  of  the  sac 
in  the  shape  of  a  tube  having  within  its  lower  end  the  intro- 
verted part,  as  the  core  on  which  it  is  moulded,  and  from  the 
apex  of  which  is  cast  off  the  substance  filling,  less  densely, 
its  interior.  The  structure  resulting  will  be  what  we  know 
as  a  hair.  Manifestly  by  progressive  enlargement  of  the  sac, 
and  further  complication  of  that  introverted  part  on  which 


302  PHYSIOLOGICAL   DEVELOPMENT. 

the  excreted  substance  is  moulded,  the  protruding  growth  may 
be  rendered  larger  and  more  involved,  as  we  see  it  in  quills 
and  feathers.  So  that  insensible  steps,  thus  indicated  in 
principle,  carry  us  from  the  exfoliation  of  epidermis  by  a  flat 
surface,  to  the  exfoliation  of  it  by  a  hollow  simple  sac,  an 
introverted  sac,  and  a  sac  further  complicated  ;  each  of  which 
produces  its  modified  kind  of  tegumentary  appendage. 

§  295.  Among  many  other  differentiations  of  the  outer 
tissues,  the  most  worthy  to  be  noticed  in  the  space  that  re- 
mains, are  those  by  which  organs  of  sense  are  formed.  We 
will  begin  with  the  simplest  and  most  closely  allied  to  the 
foregoing. 

Every  hair  that  is  not  too  long  or  flexible  to  convey  to  its 
rooted  end  a  strain  put  upon  its  free  end,  is  a  rudimentary 
tactual  organ ;  as  may  be  readily  proved  by  touching  one  of 
those  growing  on  the  back  of  the  hand.  If,  then,  a  creature 
has  certain  hairs  so  placed  that  they  are  habitually  touched 
by  the  objects  with  which  it  deals,  or  amid  which  it  moves, 
an  advantage  is  likely  to  accrue  if  these  hairs  are  modified 
in  a  way  that  enables  them  the  better  to  transmit  the  im- 
pressions derived.  Such  modified  hairs  we  have  in  the 
vibn'ssce,  or,  as  they  are  commonly  called,  the  "  whiskers " 
possessed  by  Cats  and  feline  animals  generally,  as  well  as  by 
Seals  and  many  Rodents.  These  hairs  are  long  enough  to 
reach  objects  at  considerable  distances  ;  they  are  so  stiff  that 
forces  applied  to  their  free  ends,  cause  movements  of  their 
imbedded  ends  ;  and  the  sacs  containing  their  imbedded  ends 
being  well  covered  with  nerve-fibres,  these  developed  hairs 
serve  as  instruments  of  exploration.  By  constant  use  of  them 
the  animal  learns  to  judge  of  the  relative  positions  of  objects 
past  which,  or  towards  which,  it  is  moving.  When  stealthily 
approaching  prey  or  stealthily  escaping  enemies,  such  aids  to 
perception  are  obviously  important :  indeed  their  importance 
has  been  proved  by  the  diminished  power  of  self- guidance  in 
the  dark,  that  results  from  cutting  them  o.T.  These,  then,  are 


THE    OU'IER    TISSUES    OF    AX1MALS.  3U-J 

dermal  appendages  originally  serving  the  purpose  of  cloth- 
ing, but  afterwards  differentiated  into  sense-organs. 

That  eyes  are  essentially  dermal  structures  seems  scarcely 
conceivable.    Yet  an  examination  of  their  rudimentary  types, 
and  of  their  genesis  in  creatures  that  have  them  well  deve- 
loped, shows  us  that  they  really  arise  by  successive  modifica- 
tions of  the  double  layer  composing  the  integument.     They 
make  their  first  appearance  among  the  simpler  animals  as 
specks  of  pigment,  covered  by  portions  of  epidermis  slightly 
convex  and  a  little  more  transparent  than  that  around  it. 
Here  their  fundamental  community  of   structure  with  the 
skin  is  easy  to  trace ;  and  the  formation  of  them  by  differen- 
tiation of    it  presents  no  difficulty.  Not   so   far 
in  advance  of  these  as  much  to  obscure  the  relationship,  are 
the   eyes  which  the  Crustaceans   possess.      In   every   fish- 
monger's shop  we  may  see  that  the  eyes  of  a  Lobster  are 
carried  on  pedicles  ;  and  when  the  Lobster  casts  its  shell,  the 
outer  coat  of  each  eye,  being  continuous  with  the  epidermis 
of  its  pedicle,  is  thrown  off  along  with  the  rest  of  the  exo- 
skeleton.     This  pedicle,  which   gives   the   name  of  "  stalk- 
eyed  "  Crustacea  to  a  large  group,  is,  strange  as  it  may  seem, 
a  transformed  limb.     Otherwise  shown  by  the  homologies  of 
the  parts,  this  truth  is  made  manifest  by  those  transitional  cases 
in  which  the  original   form  of  the  limb  is  retained,  and  the 
transparent  portion  which  serves  as  a  visual  organ  is  merely 
a  prominent,  patch  on  its  under  surface,  somewhat  like  a  blister, 
spreading  a  little  up  the  sides  of  the  limb — an  arrangement 
almost   thrusting   upon  us   the   suspicion   that   an  eye  is  a 
modified  portion  of  the  skin.     That  which  the  outer  appear- 
ance suggests  is  proved  by  the  structure  within.     Beneath 
the  transparent  epidermic  layer,  there  exists  a  group  of  eyes 
of  the  kind  which  we  see  in  an  insect ;  and  these,  according 
to  a  high  authority,  are  inclosed  in  the  dermal  system.     De- 
scribing the  arrangement  of  the  parts,  M.   Milne  Edwards 
writes : — "  But  the  most  remarkable  circumstance  ia,  that  the 
large   cavity   within    which    the   whole    of    these    parallel 


801  PHYSIOLOGICAL    DEVELOPMENT. 

columns,  every  one  of  which  is  itself  a  perfect  eye,  are 
contained,  is  closed  posteriorly  by  a  membrane,  which 
appears  to  be  neither  more  nor  less  than  the  middle 
tegumentary  membrane,  pierced  for  the  passage  of  the 
optic  nerve  ;  so  that  the  ocular  chamber  at  large 
results  from  the  separation  at  a  point  of  the  two  external 
layers  of  the  general  envelope."  Thus  too  is 

it,  in  the  main,  even  with  the  highly- developed  eyes  of 
the  Vertebrata.  "  The  three  pairs  of  sensory  organs  apper- 
taining to  the  higher  senses,"  says  Prof.  Huxley — "  the  nasal 
sacs,  the  eyes,  and  the  ears — arise  as  simple  coecal  involutions 
of  the  external  integument  of  the  head  of  the  embryo. 
That  such  is  the  case,  so  far  as  the  olfactory  sacs  are  con- 
cerned, is  obvious,  and  it  is  not  difficult  to  observe  that  the 
lens  and  the  anterior  chamber  of  the  eye  are  produced  in  a 
perfectly  similar  manner.  It  is  not  so  easy  to  see  that  the 
the  labyrinth  of  the  ear  arises  in  this  way,  as  the  sac  resulting 
from  the  involution  of  the  integument  is  small,  and  remains 
open  but  a  very  short  time.  But  I  have  so  frequently  veri- 
fied Huschke's  and  Remak's  statement  that  it  does  so  arise, 
that  I  entertain  no  doubt  whatever  of  the  fact.  The  outer 
ends  of  the  olfactory  sacs  remain  open,  but  those  of  the 
ocular  and  auditorv  sacs  rapidly  close  up,  and  shut  off  their 
contents  from  all  direct  communication  with  the  exterior." 
So  that,  marvellous  as  the  fact  appears,  all  that  part  of  the 
eye  which  lies  between  its  outer  surface  and  the  back  of  the 
crystalline  lens,  is  formed  in  the  same  way  as  an  ordinary  hair- 
sac,  and  is  composed  of  homologous  parts.  The  interior  coat  is 
the  epidermic  layer,  originally  continuous  with  the  surface  of 
the  skin ;  and  only  made  discontinuous  with  it  by  closure  of 
the  sac  at  the  point  which  is  afterwards  the  centre  of  the 
cornea.  This  cornea,  or  front  wall  of  the  chamber  thus  shut 
off,  is  consequently  composed  of  a  doubled  epidermic  layer 
and  an  intermediate  layer  of  the  derma  included  in  the  fold 
of  the  integument.  The  crystalline  lens,  lying  at  the  far  side 
of  this  chamber,  h  simply  a  thickening  of  the  epidermic  layer 


r  THE   OUTER   '{ISSUES    OF    ANIMALS.  305 

lining  that  part  of  the  chamber — is  developed  from  it  in  the 
same  way  that  the  substance  of  a  hair  is  developed  from  the 
papilla  at  the  bottom  of  its  sac.  The  iris  originates  as  an 
annular  thrusting-in  of  the  walls  of  this  chamber  in  front  of 
the  crystalline  lens ;  and  between  the  two  layers  of  the  epi- 
dermic lining,  thus  folded,  comes  a  portion  of  the  derma  in 
which  muscular  fibres  eventually  arise.  Though  the  founda- 
tion of  the  part  behind  the  crystalline  lens  is  laid  by  a  hollow 
diverticulum  from  the  brain,  which  grows  outwards  to  meet  the 
inward-growing  tegumentary  sac,  yet  here,  too,  structures  be- 
longing to  the  tegumentary  system  eventually  predominate. 
For  into  this  cul-de-sac  proceeding  from  the  nervous  centre, 
there  takes  place  a  lateral  growth  of  dermal  tissue,  which,  in- 
troverting the  wall  of  the  sac,  and  presently  filling  the  whole 
cavity  of  it,  is  at  last  shut  off  by  the  closure  of  the  now 
doubled  walls  of  the  sac ;  and  out  of  this  intruding  mass  of 
dermal  tissue  the  vitreous  humour  is  formed.  That  is  to  say, 
the  eye  considered  as  an  optical  apparatus  is  wholly  produced 
by  metamorphoses  of  the  skin  :  the  only  parts  of  it  not  thus 
produced,  being  the  membranes  lying  between  the  sclerotic 
and  the  vitreous  humour,  including  those  retinal  structures 
formed  in  them.  All  is  tegumentary  save  that  which  has  to 
appreciate  the  impressions  which  the  modified  integument 
concentrates  upon  it. 

Thus,  as  Prof.  Huxley  has  somewhere  pointed  out,  there 
is  a  substantial  parallelism  between  all  the  sensory  organs  in 
their  modes  of  development :  as  there  is,  too,  between  their 
modes  of  action.  A  vibrissa  may  be  taken  as  their  common 
type.  Increased  impressibility  by  an  exteinal  stimulus, 
requires  an  increased  peripheral  expansion  of  the  nervous 
system  on  which  the  stimulus  may  fall ;  and  this  is  secured 
by  an  introvertion  of  the  integument,  forming  a  sac  on  the 
walls  of  which  a  nerve  may  ramify.  That  the  more  extended 
sensory  ana  thus  constituted  may  be  acted  upon,  there 
requires  some  apparatus  conveying  to  it  from  without  the 
appropriate  stimulus  ;  and  in  the  case  of  the  tibrissa,  this 


306  PHYSIOLOGICAL    DEVELOPMENT.  , 

apparatus  is  the  epidermic  growth  which,  under  the  form  of 
a  hair,  protrudes  from  the  sac.  And  that  the  greatest 
sensitiveness  may  be  obtained,  the  external  action  must  be 
exaggerated  or  multiplied  by  the  apparatus  which  conveys  it 
to  the  recipient  nerve  ;  as  in  the  case  of  the  vibrissa,  it  is  by 
the  development  of  a  hair  into  an  elastic  lever,  that  trans- 
forms the  slight  force  acting  through  considerable  space  on 
its  exposed  end,  into  a  greater  force  acting  through  a  smaller 
space  at  its  rooted  end.  Similarly  with  the  organs  of  the 
higher  senses  In  a  rudimentary  eye,  we  have  but  a  slight 
peripheral  expansion  of  a  nerve  to  take  cognizance  of  the 
impression  ;  and  to  concentrate  the  impression  upon  it,  there 
is  nothing  beyond  a  thickening  of  the  epidermis  into  a  lens- 
shape.  But  the  developed  eye  shows  us  a  termination  of  the 
nerve  greatly  expanded  and  divided  to  receive  the  external 
stimulus.  It  shows  us  an  introverted  portion  of  the  integu- 
ment containing  the  apparatus  by  which  the  external  stimulus 
is  conveyed  to  the  recipient  nerve.  The  structure  developed 
in  this  sac  not  only  conveys  the  stimulus,  but  also,  like  its 
homologue,  concentrates  it;  and  in  the  one  case  as  in  the 
other,  the  structure  which  does  this  is  an  epidermic  growth 
from  the  bottom  of  the  sac.  Even  with  the  ear  it  is  the  same. 
Again  we  have  an  introverted  portion  of  the  integument,  on 
the  walls  of  which  the  nerve  is  distributed.  The  otolithes 
contained  in  the  sac  thus  formed,  are  bodies  which  are  set  in 
motion  by  the  vibrations  of  the  surrounding  medium,  and 
convey  these  vibrations  in  an  exaggerated  form  to  the  nerves. 
And  though  it  is  not  alleged  that  these  otolithes  are 
developed  from  the  epidermic  lining  of  the  chamber,  yet  as, 
if  not  so  developed,  they  are  concretions  from  the  contents  of 
an  epidermic  sac,  they  must  still  be  regarded  as  epidermic 
products. 

"Whether  these  differentiations  are  due  wholly  to  indirect 
equilibration,  or  whether  direct  equilibration  has  had  a  share 
in  working  them,  are  questions  that  must  be  left  open. 
Possibly  a  short  hair  so  placed  on  a  mammal's  face  as  to  be 


THE   OUTER   TISSUES   OF   ANIMALS.  307 

habitually  touched,  may,  by  conveying  excitations  to  the 
nerves  and  vessels  at  its  root,  cause  extra  growth  of  the 
bulb  and  its  appendages,  and  so  the  development  of  a  vibrissa 
may  be  furthered.  Possibly  too,  the  light  itself,  to  which  the 
tissues  of  some  inferior  animals  are  everywhere  sensitive,  may 
aid  in  setting  up  certain  of  the  modifications  by  which  the 
nervous  parts  of  visual  organs  are  formed — producing,  as  it 
must,  the  most  powerful  effects  at  those  points  on  the  surface 
which  the  movements  of  the  animal  expose  to  the  greatest 
and  most  frequent  contrasts  of  light  and  shade ;  and  propa- 
gating from  those  points  currents  of  molecular  change  through 
the  organism.  But  it  seems  clear  that  the  complexities  of 
the  sensory  organs  are  not  thus  explicable.  They  must  have 
arisen  by  the  natural  selection  of  favourable  variations. 

§  296.  A  group  of  facts,  serving  to  elucidate  those  put 
together  in  the  several  foregoing  sections,  has  to  be  added. 
I  have  reserved  this  group  to  the  last,  partly  because  it  is 
transitional — links  the  differentiations  of  the  literally  outer 
tissues  with  those  of  the  truly  inner  tissues.  Though  physi- 
cally internal,  the  mucous  coat  of  the  alimentary  canal  has 
a  §7wst-externality  from  a  physiological  point  of  view.  As 
was  pointed  out  in  the  last  chapter,  the  skin  and  the  assimi- 
lating surface  have  this  in  common,  that  they  come  in  direct 
contact  with  matters  not  belonging  to  the  organism  ;  and 
we  saw  that  along  with  this  community  of  relation  to  alien 
substances,  there  is  a  certain  community  of  structure  and  de- 
velopment. The  like  holds  with  the  linings  of  all  internal 
cavities  and  canals  that  have  external  openings. 

The  transition  from  the  literally  outer  tissues  to  those 
(issues  that  are  intermediate  between  them  and  the  truly 
inner  tissues,  is  visible  at  all  the  orifices  of  the  body ;  where 
skin  and  mucous  membrane  are  continuous,  and  the  one 
passes  insensibly  into  the  other.  This  visible  continuity  is 
not  simply  associated  with  a  great  degree  of  morphological 
continuity,  but  also  with  a  great  degree  of  physiological  con- 


308  PHYSIOLOGICAL    DEVELOPMENT. 

tiuuity.  That  is  to  say,  these  literally  outer  and  quasi- outer 
layers  are  capable  of  rapidly  assuming  one  another's  struc- 
tures and'  functions  when  subject  to  one  another's  conditions. 
Mucous  surfaces,  normally  kept  covered,  become  skin-like  if 
exposed  to  the  air;  but  resume  more  or  less  fully  their 
normal  characters  when  restored  to  their  normal  positions. 
These  are  truths  familiar  to  pathologists.  They  continually 
meet  with  proofs  that  permanent  eversion  of  the  mucous 
membrane,  even  where  it  is  by  prolapse  of  a  part  deeply 
seated  within  the  body,  is  followed  by  an  adaptation  eventu- 
ally almost  complete:  originally  moist,  tender  to  the  touch, 
and  irritated  by  the  air,  the  surface  gradually  becomes 
covered  with  a  thick,  dry  cuticle  ;  and  is  then  scarcely  more 
sensitive  than  ordinary  integument. 

Whether  this  equilibration  between  new  outer  forces  and 
reactive  inner  forces,  which  is  thus  directly  produced  in  in- 
dividuals, is  similarly  produced  in  races,  must  remain  as  a 
question  not  to  be  answered  in  a  positive  way.  On  the  one 
hand,  we  have  the  fact  that  among  the  higher  animals  there 
are  cases  of  quasi-  outer  tissues  which  are  in  one  species 
habitually  ensheathed,  while  in  another  species  they  are  not 
ensheathed  ;  and  that  these  two  tissues,  though  unquestion- 
ably homologous,  differ  as  much  as  skin  and  mucous  mem- 
brane differ.  On  the  other  hand,  there  are  certain  analogous 
changes  of  surface,  as  on  the  abdomen  of  the  Hermit- Crab, 
which  give  warrant  to  the  supposition  that  survival  of  the 
fittest  is  the  chief  agent  in  establishing  such  differentiations ; 
since  the  abdomen  of  a  Hermit-Crab,  bathed  by  water  within 
the  shell  it  occupies,  is  not  exposed  to  physical  conditions 
that  directly  tend  to  differentiate  its  surface  from  the  surface 
of  the  thorax.  But  though  in  cases  like  this  last,  we  must 
assign  the  result  to  the  natural  selection  of  variations  arising 
incidentally ;  we  may  I  think  legitimately  assign  the  result 
to  the  immediate  action  of  changed  conditions  where,  as  in 
cases  like  the  first,  we  see  these  producing  in  the  individual, 
effects  of  the  kinds  observed  in  the  race. 


THE    OUTER   TISSUES   OF   A  MM  A  US.  309 

However  this  may  be,  the  force  of  the  general  argument 
remains  the  same.  In  these  exchanges  of  structure  and 
function  between  the  outer  and  quasi- outer  tissues,  we  get 
undeniable  proof  that  they  are  easily  differentiate.  And 
seeing  this,  we  are  enabled  the  more  clearly  to  see  how  there 
have,  in  course  of  time,  arisen  those  extreme  and  multi- 
tudinous differentiations  of  the  outer  tissues  that  have  been 
glanced  at. 


CHAPTER  VIII. 

DIFFERENTIATIONS  AMONG  THE  INNER  TISSUES  OP 
ANIMALS. 

§  297.  The  change  from  the  outside  of  the  lips  to  their 
inside,  introduces  us  to  a  new  series  of  interesting  and 
instructive  facts,  joining  on  to  those  with  which  the  last 
chapter  closed.  They  concern  the  differentiations  of  those 
coats  of  the  alimentary  canal,  which,  as  we  have  seen,  are 
physiologically  outer,  though  physically  inner. 

These  coats  are  greatly  modified  at  different  parts ;  and 
their  modifications  vary  greatly  in  different  animals.  In 
the  lower  types,  where  they  compose  a  simple  tube,  running 
from  end  to  end  of  the  body,  they  are  almost  uniform  in  their 
histological  characters ;  but  on  ascending  from  these  types, 
we  find  them  presenting  an  increasing  variety  of  minute 
structures  between  their  two  ends.  The  argument  will  be 
adequately  enforced  if  we  limit  ourselves  to  the  leading 
modifications  they  display  in  some  of  the  higher  animals. 

The  successive  parts  of  the  alimentary  canal  are  so  placed 
with  respect  to  its  contents,  that  the  physical  and  chemical 
changes  undergone  by  its  contents  while  passing  from  one 
end  to  the  other,  inevitably  tend  to  transform  its  originally 
homogeneous  surface  into  a  heterogeneous  surface.  Clearly, 
the  effect  produced  on  the  food  at  any  part  of  the  canal  by 
trituration,  by  adding  a  secretion,  or  by  absorbing  its  nutri- 
tive matters,  implies  the  delivery  of  the  food  into  the  next 
part  of  the  canal  in  a  state  more  or  less  unlike  its  previous 


THK    INXEll   TISSUES    OF    ANIMALS.  3 

states — implies  that  the  surface  with  which  it  now  comes  in 
contact  is  differently  affected  by  it  from  the  preceding  sur- 
faces— implies,  that  is,  a  differentiating  action.  To  use  con- 
crete language  ; — food  that  is  broken  down  in  the  mouth  acts 
on  the  oesophagus  and  stomach  in  a  way  unlike  that  which 
it  would  have  done  had  it  been  swallowed  whole ;  the  masti- 
cated food,  to  which  certain  solvents  or  ferments  are  added, 
becomes  to  the  intestine  a  different  substance  from  that  which 
it  must  have  otherwise  been  ;  and  the  altered  food,  resolved 
by  these  additions  into  its  proximate  principles,  cannot  have 
those  proximate  principles  absorbed  in  the  next  part  of  the 
intestine,  without  the  remoter  parts  being  affected  as  they 
would  not  have  been  in  the  absence  of  absorption.  It  is  true 
that  in  developed  alimentary  canals,  such  as  the  reasoning 
here  tacitly  assumes,  these  marked  successive  differentiations 
of  the  food  are  themselves  the  results  of  pre-established 
differentiations  in  the  successive  parts  of  the  canal.  But  it  is 
also  true  that  actions  and  reactions  like  those  here  so  definitely 
marked,  must  go  on  indefinitely  in  an  undeveloped  alimentary 
canal.  If  the  food  is  changed  at  all  in  the  course  of  its  transit, 
which  it  must  be  if  the  creature  is  to  live  by  it,  then  it 
cannot  but  act  dissimilarly  on  the  successive  tracts  of  the 
alimentary  canal,  and  cannot  but  be  dissimilarlj*  reacted  on 
by  them.  Inevitably,  therefore,  the  uniformity  of  the  surface 
must  lapse  into  greater  or  less  multiformity  :  the  differentia- 
tion of  each  part  tending  ever  to  initiate  differentiations  of 
other  parts. 

Not,  indeed,  that  the  implied  process  of  direct  equilibra- 
tion can  be  regarded  as  the  sole  process.  Indirect  equilibra- 
tion aids;  and,  doubtless,  there  are  some  of  the  modifications 
which  only  indirect  equilibration  can  accomplish.  But  we 
have  here  one  unquestionable  cause — a  cause  that  is  known 
to  work  in  individuals,  changes  of  the  kind  alleged.  Where, 
for  instance,  cancerous  disease  of  the  oesophagus  so  narrows 
the  passage  into  the  stomach  as  to  prevent  easy  descent  of 
the  food,  the  oesophagus  above  the  obstruction  becomes 
VOL.  II.  14 


312  PHYSIOLOGICAL   DEVELOPMENT. 

enlarged  into  a  kind  of  pouch  ;  and  the  inner  surface  of  this 
pouch  begins  to  secrete  juices  that  produce  in  the  food  a  kind 
of.  rude  digestion.  Again,  stricture  of  the  intestine,  when  it 
arises  gradually,  is  followed  by  hypertrophy  of  the  muscular 
coat  of  the  intestine  above  the  constricted  part :  the  ordinary 
peristaltic  movements  being  insufficient  to  force  the  food 
forwards,  and  the  lodged  food  serving  as  a  constant  stimulus 
to  contraction,  the  muscular  fibres,  habitually  more  exercised, 
become  more  bulky.  The  deduction  from  general  principles 
being  thus  inductively  enforced,  we  cannot,  I  think,  resist 
the  conclusion  that  the  direct  actions  and  reactions  between 
the  food  and  the  alimentary  canal  have  been  largely  instru- 
mental in  establishing  the  contrasts  among  its  parts.  And 
we  shall  hold  this  view  with  the  more  confidence  on  observ- 
ing how  satisfactorily,  in  pursuance  of  it,  we  are  enabled  to 
explain  one  of  the  most  striking  of  these  differentiations, 
which  we  will  take  as  a  type  of  the  class. 

The  gizzard  of  a  bird  is  an  expanded  portion  of  the  alimen- 
tary canal,  specially  fitted  to  give  the  food  that  trituration 
which  the  toothless  mouth  of  the  bird  cannot  give.  Besides 
having  a  greatly-developed  muscular  coat,  this  griuding- 
charnber  is  lined  with  a  thick,  hard  cuticle,  capable  of 
bearing  the  friction  of  the  pebbles  swallowed  to  serve  as 
grind-stones.  This  differentiation  of  the  mucous  coat  into  a 
ridged  and  tubercled  layer  of  horny  matter — a  differentiation 
which,  in  the  analogous  organs  of  certain  Mollusca,  is  carried 
to  the  extent  of  producing  from  this  membrane  bony  plates, 
and  even  teeth — varies  in  birds  of  different  kinds,  according 
.  to  their  food.  It  is  moderate  in  birds  that  feed  on  flesh  and 
fish,  and  extreme  in  granivorous  birds  and  others  that  live 
on  hard  substances.  How  does  this  immense  modification  of 
the  alimentary  canal  originate  ?  In  the  stomach 

of  a  mammal,  the  macerating  and  solvent  actions  are  united 
with  that  triturating  action  which  finishes  what  the  teeth 
have  mainly  done ;  but  in  the  bird,  unable  to  masticate,  these 
internal  functions  are  specialized,  and  while  the  crop  is  the 


THE   INNEK   TISSUES    OF    AMMALS.  318 

macerating  chamber,  the  gizzard  becomes  a  chamber  adapted 
to  triturate  more  effectually.    This  adaptation  requires  simply 
an  exaggeration   of   certain   structures    and    actions   which 
characterize  stomachs   in   general,    and,    in    a   less   degree, 
alimentary   canals  throughout   their   whole   lengths.       The 
massive  muscles  of  the  gizzard  are  simply  extreme  develop- 
ments of  the  muscular  tunic,  which  is  already  considerably 
developed  over  the  stomach,  and  incloses  also  the  oesophagus 
and  the  intestine.        The  indurated  lining    of  the  gizzard, 
thickened  into  horny  buttons  at  the  places  of  severest  pres- 
sure,   is   nothing    more    than   a   greatly   strengthened    and 
modified  epithelium.    And  the  grinding  action  of  the  gizzard 
is  but  a  specialized   form  of  that  rhythmical  contraction  bv 
which  an  ordinary  stomach  kneads  the  contained  food,  and 
which  in  the  oesophagus  effects  the  act  of  swallowing,  while 
in  the  intestine  it  becomes  the  peristaltic  motion.     Allied  as 
the  gizzard  thus  clearly  is   in  structure  and  action  to  the 
stomach  and  alimentary  canal  in  general ;  and  capable  of 
being  gradually  differentiated  from  a  stomach  where  a  grow- 
ing   habit  of    swallowing   food   unmasticated   entails   more 
trituration  to  be  performed  before  the  food  passes  the  pylorus ; 
the  question  is — Does  this  change  of  structure  arise  by  direct 
adaptation  ?     There  is  warrant  for  the  belief  that  it  does. 
Besides  such  collateral  evidence  as  that  mucous  membrane 
becomes  horny  on  the  toothless   gums  of  old   people,  when 
subject  to  continual  rough  usage,  and  that  the  muscular  coat 
of  the  intestine  thickens  where  unusual  activity  is  demanded 
of  it,  we  have  the  direct  evidence  of  experiment.     Hunter 
habituated  a  sea-gull  to  feed  upon  grain,  and  found  that  the 
lining  of  its  gizzard   became  hardened,  while  the  gizzard- 
muscles  doubled  in  thickness.     A  like  change  in  the  diet  of 
a  kite  was  followed  by  like  results.    Clearly,  if  differentiations 
so  produced  in  the  individuals  of  a  race  under  changed  habits, 
are  in  any  degree  inheritable,  a  structure  like  a  gizzard  will 
originate  through  the   direct  actions  and  reactions  between 
the  food  and  the  alimentary  canal. 


314  PHYSIOLOGICAL    1/EVELOrMENT. 

Another  case — a  very  interesting  one,  somewhat  allied  to 
this — is  presented  by  the  ruminating  animals.  Here  several 
dilatations  of  the  alimentary  canal  precede  the  true  stomach  ; 
and  in  these,  large  quantities  of  unmasticated  food  are  stored, 
to  be  afterwards  returned  to  the  mouth  and  masticated  at 
leisure.  What  conditions  have  made  this  specialization 
advantageous  ?  and  by  what  process  has  it  been  established  ? 
To  both  these  questions  the  facts  indicate  answers  which  are 
not  unsatisfactory.  Creatures  that  obtain  their 

food  very  irregularly —  now  having  more  than  they  can 
consume,  and  now  being  for  long  periods  without  any — must, 
in  the  first  place,  be  apt,  when  very  hungry,  to  eat  to  the 
extreme  limits  of  their  capacities ;  and  must,  in  the  second 
place,  profit  by  peculiarities  which  enable  them  to  compensate 
themselves  for  long  fasts,  past  and  future.  A  perch  which, 
when  its  stomach  is  full  of  young  frogs,  goes  on  filling  its 
oesophagus  also ;  or  a  trout  which,  rising  to  the  fisherman's 
fly,  proves  when  taken  off  the  hook  to  be  full  of  worms  and 
insect-larvae  up  to  the  very  mouth,  gains  by  its  ability  to  take 
in  such  unusual  supplies  of  food  when  it  meets  with  them  — 
obviously  thrives  better  than  it  would  do  could  it  never  eat 
more  than  a  stomachful.  That  this  ability  to  feed  greatly  in 
excess  of  immediate  requirement,  is  one  that  varies  in  indi- 
viduals of  the  same  race,  we  see  in  the  marked  contrast 
between  our  own  powers  in  this  respect,  and  the  powers  of 
uncivilized  men  ;  whose  fasting  and  gorging  are  to  us  so 
astonishing.  Carrying  with  us  these  considerations,  we  shall 
not  be  surprised  at  finding  dilatations  of  the  oesophagus  in 
vultures  and  eagles,  which  get  their  prey  at  long  intervals 
in  large  masses  ;  and  we  may  naturally  look  for  them  too  in 
birds  like  pigeons,  which,  coming  in  flocks  upon  occasional 
supplies  of  grain,  individually  profit  by  devouring  the 
greatest  quantity  in  a  given  time.  Now  where  the  trituration 
of  the  food  is,  as  in  these  cases,  carried  on  in  a  lower  part  of 
the  alimentary  canal,  nothing  further  is  required  than  the 
storing- chamber ;  but  for  a  mammal,  having  its  grinding 


THE    INNER   TISSUES   OF   ANIMALS.  315 

apparatus  in  its  mouth,  to  gain  by  the  habit  of  hurriedly 
swallowing  umnasticated  food,  it  must  also  have  the  habit  of 
regurgitating  the  food  for  subsequent  mastication.  This 
correlation  of  habits  with  their  answering  structures,  may,  as 
we  shall  see,  arise  in  a  very  simple  way.  The 

starting  point  of  the  explanation  is  a  familiar  fact — the  fact 
that  indigestion,  often  resulting  from  excess  of  food,  is  apt  to 
cause  that  reversed  peristaltic  action  known  as  vomiting. 
From  this  we  pass  to  the  fact,  also  within  the  experience  of 
most  persons,  that  during  slight  indigestion  the  stomach  some- 
times quietly  regurgitates  a  small  part  of  its  contents  as  far 
as  the  back  of  the  mouth — giving  an  unpleasant  acquaintance 
with  the  taste  of  the  gastric  juices.  Exceptional  facts  of  the 
same  class  help  the  argument  a  step  further.  "  There  are 
certain  individuals  who  are  capable  of  returning,  at  will,  a 
greater  or  smaller  portion  of  the  contents  of  the  digesting 
stomach  into  the  cavity  of  the  mouth.  *  *  *  In  some  of  these 
cases,  the  expulsion  of  the  food  has  required  a  violent  effort.  In 
the  majority,  it  has  been  easily  evoked  or  suppressed.  While 
in  others,  it  has  been  almost  uncontrollable  ;  or  its  non- 
occurrence  at  the  habitual  time  has  been  followed  by  a 
painful  feeling  of  fulness,  or  by  the  act  of  vomiting." 
Here  then  we  have  a  certain  physiological  action,  occa- 
sionally happening  in  most  persons  and  in  some  developed 
into  a  habit  more  or  less  pronounced  :  indigestion  being  the 
habitual  antecedent.  Suppose  then  that  gregarious 

animals,  living  on  innutritive  food  such  as  grass,  are  subject  to 
a  like  physiological  action,  and  are  capable  of  like  varia- 
tions in  the  degree  of  it.  "What  will  naturally  happen  ? 
They  wander  in  herds,  now  over  places  where  food  is  scarce 
and  now  coining  to  places  where  it  is  abundant.  Some  mas- 
ticate their  food  completely  before  swallowing  it ;  while  some 
masticate  it  incompletely.  If  an  oasis,  presently  bared  by 
their  grazing,  has  not  supplied  the  whole  herd  a  full  meal, 
then  the  individuals  which  masticate  completely  will  have 
had  less  than  those  which  masticate  incompletely — will  not 


816*  PHYSIOLOGICAL   DEVELOPMENT. 

have  had  enough.  Those  which  masticate  incompletely  and 
distend  their  stomachs  with  food  difficult  to  digest,  will  be 
liable  to  these  regurgitations ;  but  if  they  re-masticate  what 
is  thus  returned  to  the  mouth  (and  we  know  that  animals 
often  eat  again  what  they  have  vomited),  then  the  extra 
quantity  of  food  taken,  eventually  made  digestible,  will  yield 
them  more  nourishment  than  -is  obtained  by  those  which 
masticate  completely  at  first.  The  habit  initiated  in  this 
natural  way,  and  aiding  survival  when  food  is  scarce, 
will  be  apt  to  cause  modifications  of  the  alimentary 
canal.  We  know  that  dilatations  of  canals  readily  arise 
under  habitual  distensions.  We  know  that  canals  habitu- 
ally distended  become  gradually  more  tolerant  of  the 
contained  masses  that  at  first  irritated  them.  And  we  know 
that  there  commonly  take  place  adaptive  modifications  of  their 
surfaces.  Hence  if  a  habit  of  this  kind  and  the  structural 
changes  resulting  from  it,  are  in  any  degree  inheritable,  it  is 
clear  that,  increasing  in  successive  generations,  both  imme- 
diately by  the  cumulative  effect  of  repetitions  and  mediately 
by  survival  of  the  individuals  in  which  they  are  most  decided, 
they  may  go  on  until  they  end  in  the  peculiarities  which 
Ruminants  display. 

§  298.  There  are  structures  belonging  to  the  same  group 
which  cannot,  however,  be  accounted  for  in  this  way.  They 
are  the  organs  that  secrete  special  products  facilitating 
digestion — the  liver,  pancreas,  and  various  smaller  glands. 
All  these  appendages  of  the  alimentary  canal,  large  and 
independent  as  some  of  them  seem,  really  arise  by  differen- 
tiations from  its  coats.  The  primordial  liver,  as  we  see  it  in 
a  simple  animal  such  as  the  Planaria,  consists  of  nothing 
more  than  bile-cells  scattered  along  a  tract  of  the  intestinal 
surface.  Accumulation  of  these  bile-cells  is  accompanied  by 
increased  growth  of  the  surface  which  bears  them — a  growth 
which  at  first  takes  the  form  of  a  cul-de-sac,  having  an  outside 
that  projects  from  the  intestine  into  the  peri- visceral  cavity 


THE    INNER   TISSUES   OF    ANIMALS.  317 

As  the  mass  of  bile-cells  becomes  greater,  there  arise  se- 
condary lateral  cavities  opening  into  the  primary  one,  and 
through  it  into  the  intestine;  until  eventually  these  cavities 
with  their  coatings  of  bile-cells,  become  ramifying  ducts  dis- 
tributed through  the  solid  mass  we  know  as  a  liver.  How  is 
this  differentiation  caused  ? 

Before  attempting  any  answer  to  this  question,  it  is  requisite 
to  inquire  the  nature  of  bile.  Is  that  which  the  liver  throws 
into  the  intestines  a  waste  product  of  the  organic  actions  ?  or 
is  it  a  secretion  aiding  digestion  ?  or  is  it  mixture  of  these  ? 
Modern  investigations  imply  that  it  is  most  likely  the  last. 
The  liver  is  found  to  have  a  compound  function.  Bernard 
has  proved  to  the  satisfaction  of  physiologists,  that  there  goes 
on  in  it  a  formation  of  glycogen — a  substance  that  is  trans- 
formed into  sugar  before  it  leaves  the  liver  and  is  afterwards 
carried  away  by  the  blood  to  eventually  disappear  in  the  lungs. 
It  is  also  shown,  experimentall}',  that  there  are  generated  in 
the  liver  certain  biliary  acids  ;  and  by  the  aid  either  of 
these  or  of  some  other  compounds,  it  is  clear  that  bile 
renders  certain  materials  more  absorbable :  its  effect  on 
fat  is  demonstrable  out  of  the  body ;  and  the  greatly 
diminished  absorption  of  fat  from  the  food  when  the 
discharge  of  bile  into  the  intestine  is  prevented,  is  probably 
one  of  the  causes  of  that  pining  away  that  results.  But  while 
recognizing  the  fact  that  the  bile  consists  in  part  of  a 
solvent,  or  solvents,  aiding  digestion,  there  is  abundant 
evidence  that  one  element  of  it  is  an  effete  product ;  and 
probably  this  is  the  primary  element.  The  yellow-green 
Bubstance  called  biliverdine,  which  gives  its  colour  to  bile,  is 
found  in  the  blood  before  it  reaches  the  liver  ;  which  is  not 
the  case  with  the  glycogen  or  the  biliary  acids.  "  As  soon  as 
the  biliary  secretion  is  in  abeyance,"  says  Dr.  Harley,  the 
most  recent  authority  on  the  subject,  "  biliverdine  accumu- 
lates in  the  blood  (until  the  serum  is  as  it  were  completely 
saturated  with  the  pigment),  from  which  it  exudes  and  stains 
the  tissues,  and  produces  the  colour  we  term  jaundice;" 


318  PHYSIOLOGICAL   DEVELOPMENT. 

*  *  *  "  the  urine  assumes  a  saffron  tint  in  consequence  of 
the  elimination  of  the  colouring  matter  by  the  kidneys  ;"  and 
afterwards  "  the  sweat,  the  milk,  the  tears,  the  sputa"  become 
yellow.  We  have  clear  proof,  then,  that  biliverdine  is  an 
excrementitious  matter,  which,  if  not  got  rid  of  through  the 
liver,  makes  its  way  out,  to  some  extent,  through  other  or- 
gans, producing  in  them  more  or  less  derangement — itching 
of  the  skin,  and  sometimes,  in  the  kidneys,  a  secondary 
disease.  That  of  the  bile  discharged  into  the  intestine,  only 
some  components  are  re- absorbed,  is  demonstrated  by  the  fact 
that  when  injected  into  the  blood,  bile  destroys  life  in  less 
than  twenty-four  hours  ;  and  that  biliverdine  is  not  among 
the  re-absorbed  components,  is  shown  both  by  the  persistence 
of  the  colour  which  it  gives  to  the  substances  in  the  intestine, 
and  by  the  absence  of  that  jaundice  which,  if  re- absorbed, 
it  would  produce.  Hence  we  are  warranted  in  classing  bili- 
verdine as  a  waste  product.  And  considering  that  the  bile- 
cells,  where  they  first  make  their  appearance  among  animals, 
are  distinguished  by  the  colour  ascribable  to  this  substance, 
we  may  fairly  infer  that  the  excretion  of  biliverdine  is  the 
original  function  of  the  liver. 

One  further  preliminary  is  requisite.  We  must  for  a 
moment  return  to  those  physico-chemical  data,  set  down  in  the 
first  chapter  of  this  work  (§§  7—8.)  We  there  saw  that 
the  complex  and  large-atomed  colloids  which  mainly  compose 
living  organic  matter,  have  extremely  little  molecular  mo- 
bility ;  and,  consequently,  extiemely  little  power  of  diffusing 
themselves.  Whereas  we  saw  not  only  that  those  absorbed 
matters,  gaseous  and  liquid,  which  further  the  decomposition 
of  living  organic  matter,  have  very  high  diffusibilities  ;  but 
tilso  that  the  products  of  the  decomposition  are  much  more 
diffusible  than  the  components  of  living  organic  matter.  And 
we  saw  that,  as  a  consequence  of  this,  the  tissues  give  ready 
entrance  to  the  substances  that  decompose  them,  and  ready 
exit  to  the  substances  into  which  they  ai-e  decomposed.  Hence 
it,  follows  that,  primarilv,  the  escape  of  effete  matters  from  the 


THE    INNER   TISSUES   OF   ANIMALS.  319 

organism,  is  a  physical  action  parallel  to  that  which  goes  on 
among  mixed  colloids  and  crystalloids  that  are  dead  or  even 
inorganic.  Excretion  is  simply  a  specialized  form  of  this 
spontaneous  action ;  and  what  we  have  to  inquire  is, — how  the 
specialization  arises. 

Two  causes  conspire  to  establish  it.  The  first  is  that  these 
products  of  decomposition  are  diffusible  in  widely  different 
degrees.  While  the  carbonic  acid  and  water  permeate  the 
tissues  with  ease  in  all  directions,  and  escape  more  or  less 
from  all  the  exposed  surfaces,  urea,  and  other  waste  substances 
incapable  of  being  vaporized,  cannot  escape  thus  readily. 
The  second  is  that  the  different  parts  of  the  organism,  being 
subject  to  different  physical  conditions,  ai*e  from  the  outset 
sure  severally  to  favour  the  exit  of  these  various  products  of 
decomposition  in  various  degrees.  How  these  causes  must 
have  co-operated  in  localizing  the  excretions,  we  shall  see  on 
remembering  how  they  now  co-operate  in  localizing  the  sepa- 
ration of  morbid  materials.  The  characteristic  substances  of 
gout  and  rheumatism  have  their  habitual  places  of  deposit. 
Tuberculous  matter,  though  it  may  be  present  in  various 
organs,  gravitates  towards  some  much  more  than  towards 
others.  Certain  products  of  disease  are  habitually  got  rid  of 
by  the  skin,  instead  of  collecting  internally.  Mostly,  these 
have  special  parts  of  the  skin  which  they  affect  rather  than 
the  rest ;  and  there  are  those  which,  by  breaking  out  sym- 
metrically on  the  two  sides  of  the  body,  show  how  definitely 
the  places  of  their  excretion  are  determined  by  certain  favour- 
ing conditions,  which  corresponding  parts  may  be  presumed 
to  furnish  in  equal  degrees.  Further,  it  is  to  be  observed 
of  these  morbid  substances  circulating  in  the  blood,  that 
having  once  commenced  segregating  at  particular  places, 
they  tend  to  continue  segregating  at  those  places.  As- 
suming, then,  as  we  may  fairlv  do,  that  this  localization 
of  excretion,  which  we  see  continually  commencing  afresh 
with  morbid  matters,  has  always  gone  on  with  the  matters 
produced  by  the  waste  of  the  tissues,  let  us  take  a  further 


320  PHYMOLOGICAL   DEVELOPMENT. 

step,  and  ask  how  localizations  become  fixed.  Other  things 
equal,  that  which  from  its  physical  conditions  is  a  place  of 
least  resistance  to  the  exit  of  an  effete  product,  will  tend  to 
become  established  as  the  place  of  excretion  ;  since  the  rapid 
exit  of  an  effete  product  will  profit  the  organism.  Other 
things  equal,  a  place  at  which  the  excreted  matter  produces 
least  detrimental  effect  will  become  the  established  place.  If 
at  any  point  the  excreted  matter  produces  a  beneficial  effect, 
then,  other  things  equal,  natural  selection  will  determine  it 
to  this  point.  And  if  facility  of  escape  anywhere  goes  along 
with  utilization  of  the  escaping  substance,  then,  other  things 
equal,  the  excretion  will  be  there  localized  by  survival  of  the 
fittest. 

Such  being  the  conditions  of  the  problem,  let  us  ask  what 
will  happen  with  the  lining  membrane  of  the  alimentary 
canal.  This,  physiologically  considered,  is  an  external  sur- 
face ;  and  matters  thrown  off  from  it  make  their  way  out  of 
the  body.  It  is  also  a  surface  along  which  is  moving  the  food 
to  be  digested.  Now,  among  the  various  waste  products 
continually  escaping  from  the  living  tissues,  some  of  the 
more  complex  ones,  not  very  stable  in  composition,  are  likety, 
if  added  to  the  food,  to  set  up  changes  in  it.  Such  changes 
may  either  aid  or  hinder  the  preparation  of  the  food  for 
absorption.  If  an  effete  matter,  making  its  exit  through  the 
wall  of  the  intestine,  hinders  the  digestive  process,  the 
enfeeblement  and  disappearance  of  individuals  in  which  this 
happens,  will  prevent  the  intestine  from  becoming  the  esta- 
blished place  for  its  exit.  While  if  it  aids  the  digestive 
process,  the  intestine  will,  for  converse  reasons,  become  more 
and  more  the  place  to  which  its  exit  is  limited.  Equally 
manifest  is  it  that  if  there  is  one  part  of  this  alimentary  canal 
at  which,  more.than  at  any  other  part,  the  favourable  effect 
results,  this  will  become  the  place  of  excretion.  If 

from  this  general  statement  we  pass  to  the  special  case 
before  us,  we  find  our  data  to  be  these  : — The  substance  to  be 
excreted,  biliverdine,  a  waste  product  of  the  organic  actions, 


THE   INNER   TISSUES   OF   ANIMALS.  321 

is,  as  jaundice  shows  us,  capable  of  escaping  out  of  the  body 
through  all  its  surfaces,  even  in  so  differentiated  a  type  as  the 
highest  mammal ;  and  in  the  undifferentiated  types  we  may 
infer  that  the  facility  of  escape  is  nearly  the  same  through 
all  the  surfaces.  For  the  gradual  localization  of  its  escapa 
at  a  particular  part  of  the  intestinal  surface,  it  is  requisite 
only  that  either  some  disadvantage  consequent  on  its  escape 
elsewhere  should  be  avoided,  or  some  advantage  due  to  its 
effect  on  digestion  should  be  gained ;  and  this  advantage 
may  be  either  direct  or  indirect.  It  is  not  necessary  that 
the  biliverdine  should  itself  act  on  the  food  :  it  is  enough  if 
it  aids  in  the  elaboration  of  other  matters,  either  nutritive  or 
solvent.  If  its  presence  causes  or  furthers  the  formation  of 
glycogen  from  other  components  of  the  blood ;  or  if  it  sets  up 
the  complex  reactions  which  generate  the  biliary  acids  ;  these 
effects  will  suffice  to  establish,  as  the  place  of  its  excretion, 
the  place  where  these  products  are  useful.  And  once  this 
place  of  excretion  having  been  established,  the  development 
of  a  liver  is  simply  a  question  of  time  and  natural  selection. 

Whether  in  this  case,  as  well  as  in  the  cases  of  the  exclu- 
sively secreting  glands  formed  along  the  alimentary  canal  (to 
which  a  modification  of  the  foregoing  argument  is  applicable), 
any  tendency  to  localization  results  from  the  immediate  action 
of  the  local  conditions,  is  an  interesting  question.  It  in 
possible  that  the  contrasts  between  the  intra- vascular  and 
extra- vascular  liquids  at  these  places  may  be  a  factor  in  the 
differentiation,  as  in  a  case  already  dealt  with.  (§  292.) 
But  this  possibility  must  be  left  undiscussed. 

§  299.  A  differentiation  of  another  order  occurring  in  the 
alimentary  canal,  is  that  by  which  a  part  of  it  is  developed 
into  a  lateral  chamber  or  chambers,  through  which  carbonic 
acid  exhales  and  oxygen  is  absorbed.  Comparative  anatomy 
and  embryology  unite  in  showing  that  a  lung  is  formed,  just 
as  a  liver  or  other  appendage  of  the  alimentary  canal  is 
formed,  by  the  growth  of  a  hollow  bud  into  the  peri- visceral 


•3^2  PHYSIOLOGICAL   DEVELOPMENT. 

cavity,  or  space  between  the  alimentary  canal  and  the  wall  of 
the  body.  The  interior  of  this  bud  is  simply  a  cul-de-sac  of 
the  alimentary  canal,  with  the  mucous  lining  of  which  its 
own  mucous  lining  is  continuous.  And  the  development  of 
this  cul-de-sac  into  an  air-chamber,  simple  or  compound,  is 
merely  a  great  extension  of  area  in  the  internal  surface  of 
the  cul-de-sac,  along  with  that  specialization  which  fits  it 
for  excreting  and  absorbing  substances  different  from  those 
which  other  parts  of  the  mucous  surface  excrete  and 
absorb.  These  lateral  air-chambers,  universal 

among  the  higher  Vertebrafa  and  very  general  among  the 
lower,  and  everywhere  attached  to  the  alimentary  canal 
jetween  the  mouth  and  the  stomach,  have  not  in  all  cases  the 
respiratory  function.  In  most  fishes  that  have  them  they 
are  what  we  know  as  swim-bladders.  In  some  fishes  the 
cavities  of  these  swim-bladders  are  completely  shut  off  from 
(lie  alimentary  canal :  nevertheless  showing,  by  the  communi- 
cations which  they  have  with  it  during  the  embryonic  stages, 
that  they  are  originally  diwrticula  from  it.  In  other  fishes 
there  is  a  permanent  ductus  pnettmaticus,  uniting  the  cavity 
of  the  swim-bladder  with  that  of  the  gullet — the  function, 
however,  being  still  not  respiratory  in  an  appreciable  degree, 
if  at  all.  But  in  certain  still  extant  representatives  of  the 
sauroid  fishes,  as  the  Lepidosfcits,  the  air-bladder  is  "divided 
into  two  sacs  that  possess  a  cellular  structure,"  and  "  the 
trachea  which  proceeds  from  it  opens  high-up  in  the  throat, 
and  is  surrounded  with  a  glottis."  In  the  Amphibia  the 
corresponding  organs  are  chambers  over  the  surfaces  of  which 
there  are  saccular  depressions,  indicating  a  transition  towards 
the  air-cells  characterizing  lungs  ;  and  accompanying  this 
advance  we  see,  as  in  the  common  Triton,  the  habit  of  coming 
up  to  the  surface  and  taking  down  a  fresh  supply  of  air  in 
place  of  that  discharged. 

How  are  the  internal  air-chambers,  respiratory  or  non- 
respiratory,  developed  ?  Upwards  from  the  amphibian  stage, 
in  which  they  arc  partially  refilled  at  long  intervals,  there  is 


THE    IICNER   TISSUES   OF   AMMALS.  323 

no  difficulty  in  understanding  how,  by  infinitesimal  steps, 
they  pass  into  complex  and  ever-moving  lungs.  But 
how  is  the  differentiation  that  produces  them  initiated  ? 
How  comes  a  portion  of  the  internal  surface  to  be  specialized 
for  converse  with  a  medium  to  which  it  is  not  naturally 
exposed?  The  problem  appears  a  difficult  one;  but  there  is 
a  not  unsatisfactory  solution  of  it. 

When  many  gold-fish  are  kept  in  a  small  aquarium,  as 
with  thoughtless  cruelty  they  frequently  are,  they  swirn 
close  to  the  surface,  so  as  to  breathe  that  water  which  is  from 
instant  to  instant  absorbing  fresh  oxj^gen.  In  doing  this 
they  often  put  their  mouths  partly  above  the  surface,  so  that 
in  closing  them  they  take  in  bubbles  of  air;  and  sometimes 
they  may  be  seen  to  continue  doing  this — the  relief  due  to 
the  slight  extra  aeration  of  blood  so  secured,  being  the 
stimulus  to  continue.  Air  thus  taken  in  may  be  detained. 
If  a  fish  that  has  taken  in  a  bubble  turns  its  head  down- 
wards, the  bubble  will  ascend  to  the  back  of  its  mouth,  and 
there  lodge;  and  coming  within  reach  of  the  contractions  of 
the  oesophagus,  it  may  be  swallowed.  If,  then,  among  fish 
thus  naturally  led  upon  occasion  to  take  in  air-bubbles,  there 
are  any  having  slight  differences  in  the  alimentary  canal  that 
facilitate  lodgment  of  the  air,  or  slight  nervous  differences 
such  as  in  human  beings  cause  an  accidental  action  to  be- 
come "  a  trick,"  it  must  happen  that  if  an  advantage  accrues 
from  the  habitual  detention  of  air-bubbles,  those  individuals 
most  apt  to  detain  them,  will,  other  things  equal,  be  more 
likely  than  the  rest  to  survive  ;  and  by  the  survival  of 
descendants  inheriting  their  peculiarities  in  the  greatest 
degrees,  and  increasing  them,  an  established  structure  and  an 
esta.blisb.ed  habit  may  arise.  And  that  they  do  in  some 
way  arise  we  have  proof :  the  common  Loach  is  well  known 
to  swallow  air,  which  it  afterwards  discharges  loaded  with 
carbonic  acid. 

From  air   thus  swallowed   the   advantages  that   may   be 
derived  are  of  two  kinds.     In  the  first  place,  the  fish  is  made 


3-4  PHYSIOLOGICAL   DEVELOPMENT. 

specifically  lighter,  and  the  muscular  effort  needed  to  keep  it 
from  sinking  is  diminished — or,  indeed,  if  the  bubble  is  of 
the  right  size,  is  altogether  saved.  The  contrast  between  the 
movements  of  a  Goby,  which,  after  swimming  up  towards  the 
surface  falls  rapidly  to  the  bottom  on  ceasing  its  exertions, 
anJ.  the  movements  of  a  Trout,  which  remains  suspended  just 
balancing  itself  by  slight  undulations  of  its  fins,  shows  how 
great  an  economy  results  from  an  internal  float,  to  fishes  which 
seek  their  food  in  mid-water  or  at  the  surface.  Hence  the 
habit  of  swallowing  air  having  been  initiated  in  the  way 
described,  we  see  why  natural  selection  will,  in  certain  fishes, 
aid  modifications  of  the  alimentary  canal  favouring  its 
lodgment — modifications  constituting  air-sacs.  In 

the  second  place,  while  from  air  thus  lodged  in  air-sacs  thus 
developed,  the  advantage  will  be  that  of  flotation  only  if  the 
air  is  infrequently  changed  or  never  changed  ;  the  advantage 
will  be  that  of  supplementary  respiration  if  the  air-sacs  are 
from  time  to  time  partially  emptied  and  refilled.  The  re- 
quirements of  the  animal  will  determine  which  of  the  two 
functions  predominates.  Let  us  glance  at  the  different  sets 
of  conditions  under  which  these  divergent  modifications 
may  be  expected  to  arise. 

The  respiratory  development  is  not  likely  to  take  place  in 
fishes  that  inhabit  seas  or  rivers  in  which  the  supply  of 
aerated  water  never  fails  :  there  is  no  obvious  reason  why 
the  established  branchial  respiration  should  be  replaced  by  a 
pulmonic  respiration.  Indeed,  if  a  fish's  branchial  respiration 
is  adequate  to  its  needs,  a  loss  would  result  from  the  effort  of 
coming  to  the  surface  for  air ;  especially  during  those  first 
stages  of  pulmonic  development  when  the  extra  aeration 
achieved  was  but  small.  Hence  in  fishes  so  circumstanced, 
the  air-chambers  arising  in  the  way  described  would  naturally 
become  specialized  mainly  or  wholly  into  floats.  Their  con- 
tained air  being  infrequently  changed,  no  advantage  would 
arise  from  the  development  of  vascular  plexuses  over  their 
surfaces  ;  nothing  would  be  gained  by  keeping  open  the  com- 


TJIK    INNER    TISSUES    OF    ANIMALS.  32-5 

munication  between  them  and  the  alimentary  canal ;  and 
there  might  thus  eventually  result  closed  chambers  the 
gaseous  contents  of  which,  instead  of  being  obtained  from 
without,  were  secreted  from  their  walls,  as  gases  often  are 
from  mucous  membranes.  Contrariwise,  aquatic 

vertebrata  in  which  the  swallowing  of  air-bubbles,  becoming 
habitual,  had  led  to  the  formation  of  sacs  that  lodged  the 
bubbles ;  and  which  continued  to  inhabit  waters  not  always 
supplying  them  with  sufficient  oxygen  ;  might  be  expected 
to  have  the  sacs  further  developed,  and  the  practice  of  chang- 
ing the  contained  air  made  regular,  if  either  of  two  advan- 
tages resulted — either  the  advantage  of  being  able  to  live  in 
old  habitats  that  had  become  untenable  without  this  modifi- 
cation, or  the  advantage  of  being  able  to  occupy  new  habitats. 
Now  it  is  just  where  these  advantages  are  gained  that  we  see 
the  pulmonic  respiration  coming  in  aid  of  the  branchial 
respiration,  and  in  various  degrees  replacing  it.  Shallow 
waters  are  liable  to  three  changes  which  conspire  to  make 
this  supplementary  respiration  beneficial.  The  summer's  sun 
heats  them,  and  raising  the  temperatures  of  the  animals  they 
contain,  accelerates  the  circulation  in  these  animals,  exalts 
their  functional  activities,  increases  the  production  of  car- 
bonic acid,  and  thus  makes  aeration  of  the  blood  more  needful 
than  usual.  Meanwhile  the  heated  water,  instead  of  yielding 
to  the  highly  carbonized  blood  brought  to  the  branchiae  the 
usual  quantity  of  oxygen,  yields  less  than  usual ;  for  as  the  heat 
of  the  water  increases,  the  quantity  of  air  it  contains  diminishes. 
And  this  greater  demand  for  oxygen  joined  with  smaller 
supply,  pushed  to  an  extreme  where  the  water  is  nearly  all 
evaporated,  is  at  last  still  more  intensely  felt  in  consequence 
of  the  excess  of  carbonic  acid  discharged  by  the  numerous 
creatures  congregated  in  the  muddy  puddles  that  remain. 
Here,  then,  it  is,  that  the  habit  of  taking  in  air-bubbles  is 
likely  to  become  established,  and  the  organs  for  utilizing  them 
developed  ;  and  here  it  is,  accordingly,  that  we  find  all  stages 
of  the  transition  to  aerial  respiration.  The  Loach  before- 


326  PHYSIOLOGICAL   DEVELOPMENT. 

mentioned,  which  swallows  air,  frequents  small  waters  liable 
to  be  consideraoly  warmed  ;  and  the  Cuckia,  an  anomalous 
eel  shaped  fish,  which  has  vascular  air-sacs  opening  out  at  the 
back  of  the  mouth,  "is  generally  found  lurking  in  holes 
and  crevices,  on  the  muddy  banks  of  marshes  or  slow-moving 
rivers."  Still  more  significant  is  the  fact  that  the  Lcpidosiren, 
or  "  mud-fish  "  as  it  is  called  from  its  habits,  is  the  only  true 
fish  that  has  lungs.  But  it  is  among  the  Amphibia  that  we  see 
most  conspicuously  this  relation  between  the  development 
of  air-breathing  organs,  and  the  peculiarities  of  the  habitats. 
Pools,  more  or  less  dissipated  annually,  and  so  rendered  unin- 
Habitable  by  most  fishes,  are  very  generally  peopled  by  these 
transitional  types.  Just  as  we  see,  too,  that  in  various 
climates  and  in  various  kinds  of  shallow  waters,  the  supple- 
mentary aerial  respiration  is  needful  in  different  degrees  ;  so 
do  we  find  among  the  Amphibia  -many  stages  in  the  substi- 
tution of  the  one  respiration  for  the  other.  The  facts,  then, 
are  such  as  give  to  the  hypothesis  a  vraiscmblance  greater  than 
could  have  been  expected. 

The  relative  effects  of  direct  and  indirect  equilibration  in 
establishing  this  further  heterogeneity,  must,  as  in  many  other 
cases,  remain  undecided.  The  habit  of  taking  in  bubbles  is 
scarcely  interpretable  as  a  result  of  spontaneous  variation  :  we 
must  regard  it  as  arising  accidentally  during  the  effort  to 
obtain  the  most  "  aerated  water ;  as  being  persevered  in 
because  of  the  relief  obtained ;  and  as  growing  by  repetition 
into  a  tendency  bequeathed  to  offspring,  and  by  them,  or 
some  of  them,  increased  and  transmitted.  The  formation  of 
the  first  slight  modifications  of  the  alimentary  canal  favouring 
the  lodgment  of  bubbles,  is  not  to  be  thus  explained.  Some 
favourable  variation  in  the  shape  of  the  passage  must  here 
have  been  the  initial  step.  But  the  gradual  increase  of  this 
structural  modification  by  the  survival  of  individuals  in  which 
it  is  carried  furthest,  will,  I  think,  be  all  along  aided  by 
immediate  adaptation.  The  part  of  the  alimentary  canal 
previously  kept  from  the  air,  but  now  habitually  in  contact 


THE    INNER  TISSUES   OF    ANIMALS.  327 

with  the  air,  must  be  in  some  degree  modified  by  the 
action  of  the  air;  and  the  directly-produced  modification, 
increasing  in  the  individual  and  in  successive  individuals, 
cunnot  cease  until  there  is  a  complete  balance  between  the 
actions  of  the  changed  agency  and  the  changed  tissue.  It  is 
indeed  probable  that  the  growth  as  well  as  the  differentiation 
of  the  pulmonic  surface,  when  once  commenced,  will  be 
furthered  by  the  direct  process.  The  reasoning  before 
used  in  the  case  of  branchiae  (§  292)  applies  in  the  case 
of  lungs.  If  exchange  between  the  plasma  in  the  blood- 
vessels and  the  plasma  in  the  tissues  surrounding  them, 
goes  on  with  a  rapidity  that  becomes  greater  where  the 
difference  between  them  becomes  greater ;  if,  consequently, 
at  some  place  where  the  carbonized  plasma  inside  the 
blood-vessels  is  brought  close  to  an  unusually  decarbonized 
or  much  oxygenated  plasma  outside  of  the  blood-vessels,  the 
exchange  of  these  liquids  becomes  unusually  active  ;  if,  as  a 
result,  the  circulation  in  the  part  is  augmented  ;  then  it  is  to 
be  inferred  that  the  extra  nutrition  will  cause  extra  growth. 
The  surface  of  the  rudimentary  lung  will  increase  in  area  so 
long  as  the  capillary  osmose  is  much  greater  than  in  other 
parts  of  the  body  ;  and  it  will  continue  to  be  greater  until, 
by  the  extension  of  the  aerating  surface,  the  respiratory 
exchange  has  been  rendered  so  efficient  as  to  bring  down  the 
contrast  between  the  intra- vascular  and  extra- vascular  liquids 
to  a  level  with  the  contrasts  between  the  intra-vascular  and 
extra- vascular  liquids  in  other  organs.  That  is  to  say,  the 
growth  which  this  direct  action  produces,  will  go  on  until  the 
functional  efficiency  of  the  lungs  is  in  equilibrium  with  the 
functional  efficiencies  of  other  parts  throughout  the  organism. 

§  300.  We  come  now  to  differentiations  among  the  truly 
inner  tissues — the  tissues  which  have  direct  converse  neither 
with  the  environment  nor  with  the  foreign  substances  taken 
into  the  organism  from  the  environment.  These,  speaking 
broadly,  are  the  tissues  which  lie  between  the  double  layer 


PHYSIOLOGICAL   DEVELOPMENT. 

forming  the  integument  with  its  appendages,  and  the  double 
layer  forming  the  alimentary  canal  with  its  diverticula.  We 
will  take  first  the  differentiation  which  produces  the  vascular 
system. 

Certain  forces  producing  and  aiding  distribution  of  liquids  in 
1  animals,  come  into  play  befoi-e  any  vascular  system  exists  ; 
and  continue  to  further  circulation  after  the  development  of 
a  vascular  system.  The  first  of  these  is  osmotic  exchange, 
acting  locally  and  having  an  indirect  general  action  ;  the 
second  is  osmotic  distension,  acting  generally  and  having  an 
indirect  local  action;  the  third  is  local  variation  of  pressure 
which  movement  of  the  body  throws  on  the  tissues  and  their 
contained  liquids.  A  few  words  are  needed  in  elucidation  of 
each.  If  in  any  creature,  however  simple,  different 

changes  are  going  on  in  parts  that  are  differently  conditioned 
— if,  as  in  &  Hydra,  one  surface  is  exposed  to  the  surrounding 
medium  while  the  other  surface  is  exposed  to  dissolved  food  ; 
then  between  the  unlike  liquids  which  the  dissimilarly-placed 
parts  contain,  osmotic  currents  must  arise ;  and  a  movement 
of  liquid  through  the  intermediate  tissue  must  go  on  as  long 
as  an  unlikeness  between  tho  liquids  is  kept  up.  This  primary 
cause  of  re-distribution  remains  one  of  the  causes  of  re-distri- 
bution in  every  more- developed  organism  :  the  passage  of 
matters  into  and  out  of  the  capillaries  is  everywhere  thus 
set  up.  And  obviously  in  producing  these  local  currents, 
osmose  must  also  indirectly  produce  general  currents,  or  aid 
them  if  otherwise  produced.  Osmose,  however,  still 

further  aids  circulation  by  the  liquid  pressure  which  it  esta- 
blishes throughout  the  organism.  More  marked  than  the 
contrasts  between  the  liquids  in  some  parts  and  those  in 
other  parts,  is  the  contrast  between  the  whole  mass  of 
liquid  in  the  animal  and  the  liquid  bathing  its  surfaces — 
either  the  water  in  which  it  is  immersed,  or  the  water  taken 
into  its  alimentary  canal.  Its  blood  and  all  its  juices  being 
denser  than  water,  the  result  is  an  osmotic  absorption  tend- 
ing ever  to  distend  all  its  permeable  parts — its  tissues, 


THE    INNER   TISSUES   OF    ANIMALS.  329 

and  its  vessels  when  it  has  them.  But  these  vessels  and 
tissues  are  elastic;  and  if  distended  must  everywhere  com- 
press their  contents — must  tend,  therefore,  to  squeeze  out  their 
contents  where  there  is  least  resistance.  Consequently,  if  at 
any  place  there  is  an  abstraction  of  nutritive  liquid,  either 
for  growth  or  function,  more  nutritive  liquid  will  be  forced 
towards  that  place.  This  cause  of  currents,  which  cannot 
fail  to  work  throughout  the  distended  tissues  even  of  animals 
th;it  are  without  blood-vessels,  comes  more  actively  into  play 
where  the  body  is  everywhere  traversed  by  these  branching 
tubes  with  elastic  walls.  When  we  learn  that  the  pressure 
of  blood  within  the  arteries  -and  veins  of  a  mammal  varies 
from  some  3  Ibs.  to  j  of  a  Ib.  per  square  inch,  we  see,  on 
averaging  this  pressure,  that  the  coats  of  the  vascular  system 
exert  considerable  force  on  the  blood.  This  average  pressure 
cannot  be  due  to  the  heart's  action ;  since  if,  in  the  absence 
of  the  heart's  action,  the  whole  mass  of  the  blood  in 
the  vascular  system  were  not  above  atmospheric  pressure, 
the  heart's  action  could  not  produce  a  pressure  above 
that  of  the  atmosphere  in  one  part  .of  the  vascular  system 
without  lowering  the  pressure  below  that  of  the  atmo- 
sphere in  another  part  of  the  vascular  system.  Hence 
it  follows  that  irrespective  of  the  heart's  action,  the  dis- 
tended walls  of  the  vascular  system  must  so*  compress 
the  blood,  as  to  cause  a  flow  of  it  to\vards  places 
where  its  escape  is  least  resisted — towards  places,  that  is, 
where  it  is  most  rapidly  abstracted  by  function  or  growth. 
This  is  a  cause  of  distribution  which  is  at  work  before  any 
central  organ  of  circulation  exists.  Though  in  the  rudimen- 
tary vascular  systems  of  the  simpler  animals,  the  osmotic 
distension  is  probably  nothing  like  so  great,  there  must 
be  some  of  it ;  and  in  the  absence  of  a  pumping  organ, 
this  force  is  probably  an  important  aid  to  that  move- 
ment of  the  blood  which  the  functions  set  up.  How 
the  third  cause — the  changes  of  internal  pressure  which  an 
animal's  movements  produce — furthers  circulation,  will  be 


&JO  PHYSIOLOGICAL    DEVELOPMENT. 

sufficiently  manifest.  That  parts  which  are  bent  or  strained 
necessarily  have  their  contained  vessels  squeezed,  has  been 
before  shown  (§  281) ;  and  whether  the  bend  or  strain  is 
caused,  as  in  a  plant,  by  an  external  force,  or,  as  usually  in  an 
animal,  by  an  internal  t'orce,  there  must  be  a  thrusting  of  the 
liquids  towards  places  of  least  resistance — that  is,  towards 
places  of  greatest  consumption.  This  which  iu  animals  with- 
out hearts  is  a  main  agent  of  circulation,  continues  to  further 
it  very  considerably  even  among-  the  highest  animals.  There 
is  experimental  proof  of  the  fact.  The  pressure  in  the  jugu- 
lar vein  of  a  horse,  which  is  about  §  of  a  pound  per 
square  inch  while  the  muscles  are  at  rest,  rises  to  2|  Ibs.  per 
square  inch  when  the  muscles  are  contracted  to  raise  the 
head.  Such,  then,  are  the  several  forces  we  have  to 

take  into  account  iu  studying  the  genesis  of  the  vascular 
system.  Let  us  now  pass  to  the  facts  to  be  interpreted. 

Even  in  such  simple  types  as  the  Hydrozoa,  cavities  in  the 
sarcode  faintly  indicate  a  structure  that  facilitates  the  transfer 
of  nutritive  matters.  These  vacuoles,  possibly  caused  by  the 
contraction  of  colloid  substance  in  passing  from  the  soluble 
to  the  insoluble  state,  become  reservoirs  filled  with  the 
plasma  that  slowly  oozes  through  the  sarcode;  and  everv 
movement  of  the  animal,  accompanied  as  it  must  be  by 
changed  pressures  and  tensions  on  these  reservoirs,  tends 
here  to  fill  them  and  there  to  squeeze  out  their  contents  in 
that  or  the  other  direction — possibly  aiding  to  produce,  by 
union  of  several  vacuoles,  those  lacunae  or  irregular  canals 
which  the  sarcode  in  some  cases  presents. 

Irregular  canals  of  this  kind,  not  lined  with  any  mem- 
branes but  being  simply  cavities  running  through  the  flesh, 
mainly  constitute  the  vascular  system  in  Molluscoida  and  many 
Mollusca.  In  the  simplest  of  these  types  the  nutritive  liquid, 
absorbed  into  the  cavity  of  the  peri- visceral  sac,  is  thrust 
hither  and  thither  through  this  sac  with  every  change  in  the 
creature's  attitude,  and  simultaneously  fills  some  of  the 
sinuses  which  open  out  of  this  sac  and  run  through  the  sub* 


THE    INNER   TISSUES   OF   ANIMALS.  331 

stance  of  the  bocty.  This  distribution  of  the  plasma,  which 
muscular  movement  and  osmotic  distension  here  combine  to 
aid,  is,  in  somewhat  more  developed  types,  further  aided  by 
a  rudimentary  heart :  in  the  peri- visceral  sac  is  seated  an 
open-mouthed  tube,  along  which  a  wave  of  contraction  pro- 
ceeds, first  for  a  while  in  one  direction  and  then  again  in  the 
opposite  direction.  The  higher  orders  of  Mollusca  have  this 
simple  contractile  tube  developed  into  a  branched  system  of 
vessels  or  arteries,  which  run  into  tbe  substance  of  the  body 
and  end  in  lacunae  or  simple  fissures.  This  ending  in  lacunas 
takes  place  at  various  distances  from  the  vascular  centre.  In 
some  genera  the  arterial  structure  is  carried  to  the  periphery 
of  the  blood-system,  while  in  others  it  stops  short  midway. 
Throughout  most  orders  of  the  Mollusca  the  back  current 
of  blood  continues  to  be  carried  by  channels  of  the  original 
kind :  there  are  no  true  veins,  but  the  blood  having  been 
delivered  into  the  tissues,  finds  its  way  back  to  the  peri-vis- 
ceral cavity  through  inosculating  sinuses.  Among  the  Ce- 
phalopods,  however,  the  afferent  blood-canals,  as  well  as  the 
efferent  ones,  acquire  distinct  walls  ;  but  even  here  the  shut- 
ting off  of  the  vascular  system  from  the  general  cavity  of  the 
body  is  not  complete;  since  there  are  still  certain  veins  which 
empty  themselves  into  the  peri-visceral  sac.  Put- 

ting together  these  facts  we  may  see  pretty  clearly  the 
stages  of  vascular  development.  From  the  original  reservoir 
of  nutritive  liquid  between  the  alimentary  canal  and  the  wall 
of  the  body,  a  portion  is  partially  shut  off;  and  by  the  ver- 
micular contraction  of  the  open  tube  thus  formed,  there  is 
produced  a  more  rapid  transfer  of  the  nutritive  liquid  from 
one  part  of  the  peri- visceral  sac  to  another,  than  was  origi- 
nally produced  by  the  motions  of  the  animal.  Clearly,  the 
extension  of  this  contractile  tube  and  the  development  from 
it  of  branches  running  hither  and  thither  into  the  tissues, 
must,  by  defining  the  channels  of  the  blood  throughout  a  part 
of  its  course,  render  its  distribution  more  regular  and  active. 
As  fast  as  this  centrifugal  growth  of  definite  channels  advances, 


332  PHYSIOLOGICAL   DEVELOPMENT. 

so  fast  are  the  efferent  currents  of  blood,  prevented  from 
escaping  laterally,  obliged  to  move  from  the  centre  towards 
the  circumference  ;  and  so  fast  also  does  the  less- developed 
set  of  channels  become,  of  necessity,  occupied  by  afferent 
currents.  When,  by  a  parallel  increase  of  definiteness,  the 
lacunae  and  irregular  sinuses  through  which  the  afferent  cur- 
rents pass,  become  transformed  into  veins,  the  accompanying 
disappearance  of  all  stagnant  or  slow-moving  collections  of 
blood,  implies  a  further  improvement  in  the  circulation. 

By  what  agency  is  effected  this  differentiation  of  a  definite 
vascular  system  from  the  indefinite  peri- visceral  sac  ?  No 
sufficient  reply  is  obvious.  The  genesis  of  the  primordial 
heart  is  not  comprehensible  as  a  result  of  direct  equilibration; 
and  we  cannot  readily  see  our  way  to  it  as  a  result  of  in- 
direct equilibration;  for  it  is  difficult  to  imagine  what  favour- 
able variation  natural  selection  could  have  seized  hold  of  to 
produce  such  a  structure.  A  contractile  tube  that  aided 
the  distribution  of  nutritive  liquid,  being  once  established, 
survival  of  the  fittest  would  suffice  for  its  gradual  extension 
and  its  successive  modifications.  But  what  were  the  early 
stages  of  the  contractile  tube,  while  it  was  yet  not  sufficiently 
formed  to  help  circulation,  and  while  it  must  nevertheless  have 
had  some  advantage  without  which  no  selective  process  could 
go  on  ?  This  part  of  the  question  we  must  leave  as  at  present 
insoluble.  To  another  part  of  the  question,  how- 

ever, an  answer  may  be  ventured.  If  we  ask  the  origin  of 
those  ramifying  channels  which,  first  appearingas  simple  chan- 
nels, eventually  become  vessels  having  definite  walls,  a  reply 
admitting  of  considerable  justification,  is,  that  the  currents  of 
nutritive  liquid  forced  and  drawn  hither  and  thither  through 
the  tissues  themselves  initiate  these  channels.  We  know  that 
streams  running  over  and  through  solid  and  quasi-solid  inor- 
ganic matter,  tend  to  excavate  definite  courses.  We  saw 
reason  for  concluding  that  the  development  of  sap-channels 
in  plants  conforms  to  this  general  principle.  May  we  not 
then  suspect  that  the  nutritive  liquid  contained  in  the  tissue 


THE   INNER   TISSUES   OF   ANIMALS.  333 

of  a  simple  animal,  made  to  ooze  now  in  this  direction  and 
now  in  that  by  osmotic  distension  and  by  the  changes  of 
pressure  which  the  animal's  movements  cause,  comes  to  have 
certain  lines  along  which  it  is  thrust  backwards  and  forwards 
more  than  along  other  lines  ;  and  must  by  repeated  passings 
make  these  more  and  more  permeable,  until  they  become 
lacunas  ?  Such  actions  will  inevitably  go  on;  and  such  actions 
appear  competent  to  produce  some,  at  least,  of  the  observed 
effects.  The  leading  facts  which  indicate  that  this  is  a  part 
cause  of  vascular  development,  are  these. 

Growths  normally  recurring  in  certain  places  at  certain 
intervals,  are  accompanied  by  local  formations  of  blood-vessels. 
The  periodic  maturation  of  ova  among  the  Mammalia,  supplies 
an  instance.  Through  the  stroma  of  an  ovarium  are  dis- 
tributed innumerable  minute  vesicles,  which,  in  their  early 
stages,  are  microscopic.  Of  these,  severally  contained  in  their 
minute  ovi-sacs,  any  one  may  develop:  the  determining 
cause  being  probably  some  slight  excess  of  nutrition.  When 
the  development  is  becoming  rapid,  the  capillaries  of  the 
neighbouring  stroma  increase  and  form  a  plexus  on  the  walla 
of  the  ovi-sac.  Now  since  there  is  no  typical  distribution  of 
the  developing  ova  ;  and  since  the  increase  of  an  ovum  to 
a  certain  size  precedes  the  increase  of  vascularity  round 
it ;  we  can  scarcely  help  concluding  that  the  setting  up 
of  currents  towards  the  point  of  growth  determines  the 
formation  of  the  blood-vessels.  It  may  be  that  having 
once  commenced,  this  local  vascular  structure  completes 
itself  in  a  typical  manner ;  but  it  seems  clear  that  this 
greater  development  of  blood-vessels  around  the  growing 
ovum  is  initiated  by  the  draught  towards  it.  Ab- 

normal growths  show  still  better  this  relation  of  cause  and  ef- 
fect. The  fake  membranes  sometimes  found  in  the  bronchial 
tubes  in  croup,  may  perhaps  fairly  be  held  abnormal  in  but  a 
partial  sense  :  it  may  be  said  that  their  vascular  systems  are 
formed  after  the  type  of  the  membranes  to  which  they  are 
akin.  But  this  can  scarcely  be  said  of  the  morbid  growths 


334  PHYSIOLOGICAL   DEVELOPMENT. 

classed  as  malignant.  The  blood-vessels  in  an  encephuloid 
cancer,  are  led  to  enlarge  and  ramify,  often  to  an  immense 
extent,  by  the  unfolding  of  the  morbid  mass  to  which  they 
carry  blood.  Alien  as  is  the  structure  as  a  whole  to  the  type 
of  the  organism  ;  and  alien  in  great  measure  as  is  its  tissue 
to  the  tissue  on  which  it  is  seated  ;  it  nevertheless  happens 
that  the  growth  of  the  alien  tissue  and  accompanying  ab- 
straction of  materials  from  the  blood-vessels,  determine  a 
corresponding  growth  of  these  blood-vessels.  Unless,  then, 
we  say  that  there  is  a  providentially  created  type  of  vascular 
structure  for  each  kind  of  morbid  growth  (and  even  this 
would  not  much  help  us,  since  the  vascular  structure  has 
no  constancy  within  the  limits  of  each  kind),  we  are  com- 
pelled to  admit  that  in  some  way  or  other  the  currents  of 
blood  are  here  directly  instrumental  in  forming  their  own 
channels.  One  more  piece  of  evidence,  before  cited 

as  exemplifying  adaptation  (§  67),  may  be  called  to  mind. 
When  any  main  channel  for  blood,  leading  to  or  from  a 
certain  part  of  the  body,  has  been  rendered  impervious, 
others  among  the  channels  leading  to  or  from  this  same  part, 
enlarge  to  the  extent  requisite  for  fulfilling  the  extra  func- 
tion that  falls  upon  them  :  the  enlargement  being  caused,  as 
we  must  infer,  by  the  increase  of  the  currents  carried. 

Here  then  are  facts  warranting  inductively  the  deduction 
above  drawn.     It  is  true  that  we  are  left  in  the  dark  respect- 
ing the  complexities  of  the  process.     How  the  channels  fur 
blood  come  to  have  limiting  membranes,  and  many  of  them 
muscular  coats,  the  hypothesis  does  not  help  us  to  say.      But 
the  evidence  assigned  goes  far  to  warrant  the  belief  that  vascu- 
lar development  is  initiated  by  direct  equilibration  ;  though  in 
direct  equilibration  may  have  had  the  larger  share  in  establish 
ing  the  structures  which  distinguish  finished  vascular  systems 

§  301.  Of  the  inner  tissues  which  remain  let  us  next  take 
bone.  In  what  manner  is  differentiated  this  dense  substance 
serving  in  most  cases  for  internal  support  ? 


THE    INNER   TISSUES   OF   ANIMALS.  335 

Already  when  considering  the  vertebrate  skeleton  under 
its  morphological  aspect  (§  256)  it  was  pointed  out  that  the 
formation  of  dense  tissues,  internal  as  well  as  external,  is,  in 
some  cases  at  least,  brought  about  by  the  mechanical  forces 
to  be  resisted.  Through  what  process  it  is  brought  about  we 
could  not  then  stay  to  inquire :  this  question  being  not 
morphological  but  physiological.  Answers  to  some  kindred 
questions  have  since  been  attempted.  Certain  actions  to 
which  the  internal  dense  tissues  of  plants  may  be  ascribed, 
have  been  indicated  ;  and  more  recently,  analogous  actions 
have  been  assigned  as  causes  of  spme  external  dense  tissues 
of  animals.  We  have  now  to  ask  whether  actions  of  the 
same  nature  have  produced  these  internal  dense  tissues  of 
animals. 

The  problem  is  an  involved  one.  Bones  have  more  than  one 
stage:  they  are  membranous  or  cartilaginous  before  they  be- 
come osseous ;  and  their  successive  component  substances  so  far 
differ  that  the  effects  of  mechanical  actions  upon  them  differ. 
And  having  to  deal  with  transitional  states  in  which  bone  is 
formed  of  mixed  tissues,  having  unlike  physical  properties 
and  unlike  minute  structures,  the  effects  of  strains  become 
too  complicated  to  follow  with  precision.  Anything  in  the 
way  of  interpretation  must  therefore  be  regarded  as  tentative. 
If  analysis  and  comparison  show  that  the  phenomena  are  not 
inconsistent  with  the  hypothesis  of  mechanical  genesis,  it  is 
as  much  as  can  be  expected.  Let  us  first  observe  more  nearly 
the  mechanical  conditions  to  which  bones  are  subject. 

The  endo-skeleton  of  a  mammal  with  the  muscles  and  liga- 
ments holding  it  together,  may  be  rudely  compared  to  a, 
structure  built  up  of  struts  and  ties  ;  of  which,  speaking 
generally,  the  struts  bear  the  pressures  and  the  ties  bear  the 
tensions.  The  framework  of  an  ordinary  iron  roof  will  give 
an  idea  of  the  functions  of  these  two  elements,  and  of  the 
mechanical  characters  required  by  them.  Such  a  framework 
consists  partly  of  pieces  that  have  each  to  bear  a  thrust  in 
the  direction  of  its  length,  and  partly  of  pieces  that  have  each 
Vot.  II.  is 


&36  PHYSIOLOGICAL    DEVELOPMENT. 

to  bear  a  pull  in  the  direction  of  its  length ;  and  these  struts 
and   ties   are   differently    formed    to    adapt    them   to   these 
different  strains.    Further,  it  should  be  remarked  that  though 
fue  rigidity  of  the  framework  depends  on  the  ties  which  are 
flexible,  as  much  as  on  the  struts  which  are  stiff,  yet  the  ties 
help  to  give  the  rigidity  simply  by  so  holding  the  struts  in 
position  that  they  cannot  escape  from  the  thrusts  which  fall 
on   them.     Now  the  like   relation   holds  with  a  difference 
among  the  bones  and  muscles — the  difference  being,  that  here 
the  ties  admit  of  being  lengthened  or  shortened  and  the  struts 
of  being  moved  about  upon  their  joints.     The  mechanical  re- 
lations are  not  altered  by  this  however.     The  actions  are  of 
essentially  the  same  kind  in  an  animal  that  is  standing,  or 
keeping  itself  in  a  strained  attitude,  as  in  one  that  is  changing 
its  attitude — the  same  in  so  far  that  we  have  in  each  a  set 
of  flexible  parts  that  are  pulling  and  a  set  of  rigid  parts  that 
are  resisting.     It  needs  but  to  remember  the  sudden  collapse 
and  fall  that  take  place  when  the  muscles  are  paralyzed,  or 
to  remember  the  inability  of  a  bare  skeleton  to  support  itself, 
to  see  that  the  struts  without  the  ties  cannot  suffice.     And 
we  have  but  to  think  of  the  formless  mass  into  which  a  man 
would  sink  when  deprived  of  his  bones,  to  see  that  the  ties 
without  the  struts  cannot  suffice.     To  trace  the  way  in  which 
a  particular  bone  has  its  particular  thrust  thrown  upon  it, 
may  not  always  be  practicable.     Though  it  is  easy  to  perceive 
how  a  flexor  or  extensor  of  the  arm  causes  by  its  tension  a  re- 
active pressure  along  the  line  of  the  humerus,  and  is  enabled 
to  produce  its  effect  only  by  the  rigidity  of  the  humerus ;  yet 
it  is  not  so  easy  to  perceive  how  such  bones  as  those  of  a 
horse's  haunch  are  similarly  acted  upon.     Still,  as  the  weight 
of  the  hind  quarters  has  to  be  transferred  from  the  pelvis  to 
the  feet,  and  must  be  so  transferred  through  the  bones,  it  is 
manifest  that  though  these  bones  form  a  very  crooked  line, 
the  weight  must  produce  a  pressure  along  the  axis  of  each  :  the 
muscles  and  ligaments  concerned  serving  here,  as  in  other 
cases,  so  to  hold  the  bones  that  they  bear  the  pressure  instead 


THE    IXNEll   TISSUES    OF    ANIMALS.  337 

of  being  displaced  by  it.  Not  forgetting  that  many  processes 
of  the  bones  have  to  bear  tensions,  we  may  then  say  thut 
generally,  though  by  no  means  universally,  bones  are  in- 
ternal dense  masses  that  have  to  bear  pressures — pressures 
which  in  the  cylindrical  bones  become  longitudinal  thrusts. 
Leaving  out  exceptional  cases,  let  us  consider  bones  as  masses 
thus  circumstanced. 

When  giving  reasons  for  the  belief  that  the  vertebrate 
skeleton  is  mechanically  originated,  one  of  the  facts  put  in 
evidence  was,  that  in  the  vertebrate  series  the  transition  from 
the  cartilaginous  to  the  osseous  spine  begins  peripherally 
(§  257)  :  each  vertebra  being  at  first  a  ring  of  bone  sur- 
rounding a  mass  of  cartilage.  And  it  was  pointed  out  that 
this  peripheral  ossification  is  ossification  at  the  region  ot 
greatest  pressures.  Now  it  is  not  vertebra)  only  that  follow 
this  course  of  development.  In  a  cylindrical  bone,  though 
it  is  differently  circumstanced,  the  places  of  commencing  ossi- 
fication are  still  the  places  on  which  the  severest  stress  falls. 
Let  us  consider  how  such  a  bone  that  has  to  bear  a  longitu- 
dinal pressure  is  mechanically  affected.  If  the  end  of  a 
walking-cane  be  thrust  with  force  against  the  ground,  the  cane 
bends ;  and  partially  resuming  its  straightness  when  relieved, 
again  bends,  usually  towards  the  same  side,  when  the  thrust 
is  renewed.  A  bend  so  caused  acts  on  the  fibres  of  the  cane 
in  nearly  the  same  way  as  does  a  bend  caused  by  supporting 
the  cane  horizontally  at  its  two  ends  and  suspending  u 
weight  from  its  middle.  In  either  case  the  fibres  on  the  con- 
vex side  are  extended  and  the  fibres  on  the  concave  side  com- 
pressed. Kindred  actions  occur  in  a  rod  that  is  so  thick  as 
not  to  yield  visibly  under  the  force  applied.  In  the  absence 
of  complete  homogeneity  of  its  substance,  complete  symmetry 
in  its  form,  and  an  application  of  a  force  exactly  along  its 
axis,  there  must  be  some  lateral  deflection ;  and  therefore 
some  distribution  of  tensions  and  pressures  of  the  kind  indi- 
cated. And  then,  as  the  fact  which  here  specially  concerns  us, 
we  have  to  note  that  the  strongest  tensions  and  pressures  are 


33S  PHYSIOLOGICAL    DEVELOPMENT. 

borne  by  the  outer  layers  of  fibres.  Now  the  shaft  of  a  long 
bone,  subject  to  mechanical  actions  of  this  kind,  similarly  has 
its  outer  layer  most  strained.  In  this  layer,  therefore,  on  the 
mechanical  hypothesis,  ossification  should  commence,  and  here 
it  does  commence — commences,  too,  midway  between  the  ends 
where  the  bends  produce  on  the  superficial  parts  their  most 
intense  effects.  But  we  have  not  in  this  place  simply 

to  observe  that  ossification  commences  at  the  places  of  greatest 
stress,  but  to  ask  what  causes  it  to  do  this.  Can  we  trace  the 
physical  actions  which  set  up  this  deposit  of  dense  tissue  ?  It 
is,  I  think,  possible  to  indicate  a  "  true  cause  "  that  is  at  work ; 
though  whether  it  is  a  sufficient  cause  may  be  questioned. 
We  concluded  that  in  certain  other  cases,  the  formation  of 
dense  tissue  indirectly  results  from  the  alternate  squeezing 
and  relaxation  of  the  vessels  running  through  the  part ;  and 
the  inquiry  now  to  be  made  is,  whether,  in  developing  bone, 
the  same  actions  go  on  in  such  ways  as  to  produce  the  ob- 
served effects.  At  the  outset  we  are  met  by  what  seems  a 
fatal  difficulty — cartilage  is  a  non- vascular  tissue :  this  sub- 
stance of  which  unossified  bones  consist  is  not  permeated  by 
minute  canals  carrying  nutritive  liquid,  and  cannot,  there- 
fore, be  a  seat  of  actions  such  as  those  assigned.  This  ap- 
parent difficulty,  however,  furnishes  a  confirmation.  For 
cartilage  that  is  wholly  without  blood-vessels  does  not  ossify : 
ossification  takes  place  only  at  those  parts  of  it  into 
which  the  capillaries  penetrate.  Hence,  we  get  additional 
reason  for  suspecting  that  bone- formation  is  due  to  the  alleged 
cause  ;  since  it  occurs  where  mechanical  strains  can  produce 
the  actions  described,  but  does  not  occur  where  mechanical 
strains  cannot  produce  them.  Let  us  consider  more  closely 
what  the  factors  are,  and  how  they  will  cooperate  under 
the  particular  conditions.  It  seems  possible  that 

these  canals  that  exist  in  the  superficial  layer  of  a  cartilagin- 
ous bone  before  it  begins  to  ossify,  are  themselves  produced 
by  the  mechanical  actions.  For  every  time  a  mass  of  carti- 
lage is  strained  and  its  superficial  layers  more  especially 


THE    INNER   TISSUES   OF   ANIMALS.  339 

subject  to  tensions  and  pressures,  the  nutritive  liquid  diffused 
through  the  substance  of  the  cartilage,  compressed  as  it  must 
be,  will  tend  to  ooze  from  the  surface  of  the  cartilage,  and  to 
return  again  when  the  stress  is  taken  off.  Such  alternate 
movements  of  the  nutritive  liquid,  perpetually  repeated,  will 
be  apt  to  form  channels.  These,  at  first  quite  superficial  and 
inappreciable,  will  become  more  appreciable  ;  since,  when 
they  are  once  commenced,  any  further  additions  of  substance 
to  the  surface  will  be  prevented  from  closing  their  openings 
by  the  alternate  rushes  of  liquid ;  and  so  a  vascular  layer 
of  appreciable  thickness  may  gradually  be  formed.  But 
without  doing  more  than  hint  this,  it  will  suffice  for  the 
argument  if  we  commence  with  the  external  vascular  layer 
as  already  existing,  and  consider  what  will  take  place  in 
it.  Cartilage  is  elastic — is  somewhat  extensible,  and 

spreads  out  laterally  under  pressure,  but  resumes  its  form 
when  relieved.  How,  then,  will  the  capillaries  traversing 
such  a  substance  be  affected  at  the  places  where  it  is  strained 
by  a  bend  ?  Those  on  the  convex  side  will  be  laterally 
squeezed,  in  the  same  way  that  we  saw  the  sap- vessels  on  the 
convex  side  of  a  bent  branch  are  squeezed  ;  and  as  exudation 
of  the  sap  into  the  adjacent  prosenchyma  will  be  caused  in 
the  one  case,  so,  in  the  other,  there  will  be  caused  exudation 
of  serum  into  the  adjacent  cartilage :  extra  nutrition  and 
increase  of  strength  resulting  in  both  cases.  The  parallel 
ceases  here,  however.  In  the  shoot  of  a  plant,  bent  in 
various  directions  by  the  wind,  the  side  which  was  lately 
compressed,  is  now  extended ;  and  hence  that  squeezing 
of  the  sap-vessels  which  results  from  extension,  suffices  to 
feed  and  harden  the  tissue  on  all  sides  of  the  shoot.  But  it 
is  not  so  with  a  bone.  Having  yielded  on  one  side  under 
longitudinal  pressure,  and  resumed  as  nearly  as  may  be  its 
previous  shape  when  the  pressure  is  taken  off,  the  bone  yields 
again  towards  the  same  side  when  again  longitudinally 
pressed.  Hence  the  substance  of  its  concave  side,  never 
rendered  convex  by  a  bend  in  the  opposite  direction,  would 


310  I'HYrflO LOGICAL   DEVELOPMENT. 

not  receive  any  extra  nutrition  did  no  other  action  come  into 
play.  But  if  we  consider  how  intermittent  pressures  must 
act  on  cartilage,  we  shall  see  that  there  will  result  extra 
nutrition  of  the  concave  side  also.  Squeeze  between  two 
pieces  of  glass  a  thin  bit  of  caoutchouc  that  has  a  hole 
through  it.  While  the  caoutchouc  spreads  out  away  from 
the  centre,  it  also  spreads  inwards,  so  as  partially  to  close  the 
hole.  Everywhere  its  molecules  move  away  in  directions  of 
least  resistance ;  and  for  those  near  the  hole,  the  direction  of 
least  resistance  is  towards  the  hole.  Let  this  hole  stand  for 
the  transverse  section  of  one  of  the  capillaries  passing 
through  cartilage,  and  it  will  be  manifest  that  on  the  side  of 
the  unossitied  bone  made  concave  in  the  way  described,  the 
compressed  cartilage  will  squeeze  the  capillaries  traversing 
it;  and  in  the  absence  of  perfect  homogeneity  in  the 
cartilage,  the  squeeze  will  cause  extra  exudation  from  the 
capillaries  into  the  cartilage.  Thus  every  additional  strain 
will  give  to  the  cartilage  it  falls  upon,  an  additional  supply 
of  the  materials  for  growth.  So  that  presently  the  side 
which,  by  yielding  more  than  any  other,  proves  itself  to  be 
the  weakest,  will  cease  to  be  the  weakest.  What  further  will 
happen  ?  Some  other  side  will  yield  a  little — the  bends  will 
take  place  in  some  other  plane ;  and  the  portions  of  cartilage 
on  which  repeated  tensions  and  pressures  now  fall  will  be 
strengthened.  Thus  the  rate  of  nutrition,  greatest  at  the 
place  where  the  bending  is  greatest,  and  changing  as  the 
incidence  of  forces  changes,  will  bring  about  at  every  point  a 
balance  between  the  resistances  and  the  strains.  Thus,  too, 
there  will  be  determined  that  peripheral  induration  which  we 
see  in  bones  so  circumstanced.  As  in  a  shoot  we  saw  that  the 
woody  deposit  takes  place  towards  the  outside  of  the  cylinder, 
where,  according  to  the  hypothesis,  it  ought  to  take  place ; 
so,  here,  we  see  that  the  excess  of  exudation  and  hardening, 
occurring  where  the  strains  are  most  intense,  will  form  a 
cylinder  having  a  dense  outside  and  a  porous  or  hollow 
inside.  These  processes  will  bo  essentially  the  same 


THE    IS  NEK   TISSUES   OF    ANIMALS.  34.1 

in  Lones  subject  to  more  complex  mechanical  actions ;  such 
as  sundry  of  the  flat  bones  and  others  that  serve  as  internal 
fulcra.  Be  the  strains  transverse  or  longitudinal,  be  they 
torsion  strains  or  mixed  strains,  the  outer  parts  of  the  bone 
will  be  more  affected  by  them  than  its  inner  parts.  They 
will  therefore  tend  everywhere  to  produce  resisting  masses 
having  outer  parts  more  dense  than  their  inner  parts.  And 
by  causing  most  growth  where  they  are  most  intense,  will 
call  out  reactive  forces  adequate  to  balance  them — forms  and 
thicknesses  of  bone  offering  resistances  equal  to  the  strains, 
however  numerous  and  varied.  There  are  doubt- 

less obstacles  in  the  way  of  this  interpretation.  It  may  be 
said  that  the  forces  acting  on  the  outer  layers  in  the  manner 
described,  would  compress  the  capillaries  too  little  to  produce 
the  alleged  effects  ;  and  if  evenly  distributed  along  the  whole 
lengths  of  the  layers,  they  would  probably  be  so.  But  it 
needs  only  to  bend  a  flexible  mass  and  observe  the  tendency 
to  form  creases  on  the  concave  surface,  to  feel  assured  that 
along  the  surface  of  an  ossifying  bone,  the  yielding  of  the 
tissue  when  bent  will  not  be  uniform.  In  the  absence  of 
complete  homogeneity,  the  interstitial  yielding  will  take 
place  at  some  points  more  than  others,  and  at  one  point  above 
all  others.  At  these  weakest  points,  and  especially  at  one, 
the  action  on  the  capillaries  will  be  concentrated.  When, 
at  the  weakest  point — the  centre  of  commencing  ossification 
— an  extra  amount  of  deposit  has  been  caused,  it  will  cease 
to  be  the  weakest ;  and  adjacent  points,  now  the  weakest,  will 
become  the  places  of  yielding  and  induration.  And  in  pro- 
portion as  the  layer  becomes  filled  with  unyielding  matter, 
the  remaining  compressible  parts  of  it,  and  their  contained 
capillaries,  will  be  more  severely  compressed.  It  may  be 
further  objected  that  the  hypothesis  is  incompatible  with  the 
persistence  of  cartilage  for  so  long  a  time  between  the 
epiphyses  of  bones  and  the  bony  masses  which  they  ter- 
minate. But  there  is  the  reply  that  the  places  occupied  by 
this  cartilage,  being  places  at  which  the  bone  lengthens,  the 


342  PHYSIOLOGICAL   DEVELOPMENT. 

non- ossification  is  in  part  apparent  only — it  is  rather  that 
new  cartilage  is  formed  as  fasD  as  the  pre-existing  cartilnge 
ossifies  ;  and  there  is  the  further  reply  that  the  slowness  of 
the  ultimate  ossification  of  this  part,  is  due  to  its  non- 
vascularity,  and  to  mechanical  conditions  that  are  unfavour- 
able to  its  acquirement  of  vascularity.  Once  more,  the  de- 
murrer that  in  the  epiphyses  ossification  does  not  begin  at 
the  surface  but  within  the  mass  of  the  cartilage,  is  met  by  an 
explanation  parallel  to  that  before  given  (§  293,  note)  of  the 
deep-seated  induration  produced  by  an  external  pressure 
which,  during  long  intervals,  does  not  intermit  completely ; 
as  in  a  bunion,  a  node  on  the  instep,  and  what  is  called 
"  housemaid's  knee." 

Of  course  it  is  not  meant  that  this  osseous  development  by 
direct  equilibration,  takes  place  in  the  individual.  Though 
it  is  a  corollary  from  the  argument  that  in  each  individual 
the  process  must  be  furthered  and  modified  by  the  particular 
actions  to  which  the  particular  bones  are  exposed ;  yet  the 
leading  traits  of  structure  assumed  by  the  bones  are  assumed 
in  conformity  with  the  inherited  type.  This,  however,  is  no 
difficulty.  The  type  itself  is  to  be  regarded  as  the  accumulated 
result  of  such  modifications,  transmitted  and  increased  from 
generation  to  generation.  The  actions  above  described  as 
taking  place  in  the  bone  of  an  individual,  must  be  understood 
as  producing  their  total  effect  little  by  little  in  the  corre- 
sponding bones  of  a  long  series  of  individuals.  Even  if  but 
a  small  modification  can  be  so  wrought  in  the  individual,  yet 
if  such  modification,  or  a  part  of  it,  is  inheritable,  we  may 
readily  understand  how,  in  the  course  of  geologic  epochs,  the 
observed  structures  may  arise  by  the  assigned  way. 

Here  may  fitly  come  in  a  strong  confirmation.  If  we  find 
cases  where  individual  bones,  subject  in  exceptional  degrees 
to  the  actions  described,  present  in  exceptional  amounts  the 
modifications  attributed  to  them,  we  are  greatly  helped  in 
understanding  how  there  may  be  produced  in  the  race  that 
aggregate  of  modifications  which  the  hypothesis  implies. 


THE    INKER   TISSUES   OF   ANIMALS.  343 

fiueh  cases  occur  in  picket ty  children.  I  am  indebted  to  Mr. 
Busk  for  pointing  out  these  abnormal  formations  of  dense 
tissue,  that  are  not  apparently  explicable  as  results  ot 
mechanical  actions  and  re-actions.  It  was  only  on  tracing 
out  the  processes  here  at  work,  that  there  suggested  itself  the 
specific  interpretation  of  the  normal  process,  as  above  set 
forth.  "When,  from  constitutional  defect,  bones  do 

not  ossify  with  due  rapidity,  and  are  meanwhile  subject  to 
the  ordinary  strains,  they  become  distorted.  Remembering 
how  a  mass  which  has  been  made  to  yield  in  any  direction 
by  a  force  it  cannot  withstand,  is  some  little  time  before  it 
recovers  completely  its  previous  form,  and  usually,  indeed, 
undergoes  what  is  called  a  "  permanent  set ;"  it  is  inferable 
that  when  a  bone  is  repeatedly  bent  at  the  same  time  that 
the  liquid  contained  in  its  capillaries  is  poor  in  the  materials 
for  forming  dense  tissue,  there  will  not  take  place  a  propor- 
tionate strengthening  of  the  parts  most  strained  ;  and  these 
parts  will  give  v?ay.  This  happens  in  rickets.  But  this 
having  happened,  there  goes  on  what,  in  teleological  language, 
we  call  a  remedial  process.  Supposing  the  bone  to  be  one 
commonly  affected — a  femur  ;  and  supposing  a  permanent 
bend  to  have  been  caused  in  it  by  the  weight  of  the  body  ; 
the  subsequent  result  is  an  unusual  deposition  of  cartilaginous 
and  osseous  matter  on  the  concave  side  of  the  bone.  If  the 
bone  is  represented  by  a  strung  bow,  then  the  deposit  occurs 
at  the  part  represented  by  the  space  between  the  bow  and 
the  string.  And  thus  occurring  where  its  resistance  is  most 
effective,  it  increases  until  the  approximately-straight  piece 
of  bone  formed  within  the  arc,  has  become  strong  enough  to 
bear  the  pressure  without  appreciably  yielding.  Now 

this  direct  adaptation,  seeming  so  like  a  special  provision, 
and  furnishing  so  remarkable  an  instance  of  what,  in  medical 
but  unscientific  language,  is  called  the  vts  inedicatrix  nafiira1, 
is  simply  a  result  of  the  above-described  mechanical  actions 
and  re-actions,  going  on  under  the  exceptional  conditions. 
Each  time  such  a  bent  bone  is  subject  to  a  force  which  ag.iiu 


344  PHYSIOLOGICAL    DEVELOPMENT. 

bends  it,  the  severest  compression  fulls  on  the  substance  of 
its  concave  side.  Each  time,  then,  the  capillaries  running 
through  this  part  of  its  substance  are  violently  squeezed — 
far  more  squeezed  than  they  or  any  other  of  the  capillaries 
would  have  been,  had  the  bone  remained  straight.  Hence, 
on  every  repetition  of  the  strain,  these  capillaries  near  the 
concave  surface  have  their  contents  forced  out  in  more 
than  normal  abundance.  The  materials  for  the  formation  of 
tissue  are  supplied  in  quantity  greater  than  can  be  assimi- 
lated by  the  tissue  already  formed ;  and  from  the  excess  of 
exuded  plasma,  new  tissue  arises.  A  layer  of  organizable 
material  accumulates  between  the  concave  surface  and  the 
periosteum ;  in  this,  according  to  the  ordinary  course  of 
tissue-growth,  new  capillaries  appear ;  and  the  added  layer 
presently  assumes  the  histological  character  of  the  layer  from 
which  it  has  grown.  What  next  happens  ?  This  added 
layer,  further  from  the  neutral  axis  than  that  which  has 
thrown  it  out,  is  now  the  most  severely  compressed,  and  its 
capillaries  are  the  most  severely  squeezed.  The  place  of 
greatest  exudation  and  most  rapid  deposit  of  matter,  is  there- 
lore  transferred  to  this  new  layer ;  and  at  the  same  time  that 
active  nutrition  increases  its  density,  the  excess  of  organizable 
material  forms  another  layer  external  to  it :  the  successive 
layers  so  added,  encroaching  on  the  space  between  the  concave 
surface  of  the  bone  and  the  chord  of  its  arc.  What 

limits  the  encroachment  on  this  space  ? — what  stops  the  pro- 
cess of  filling  it  up  ?  The  answer  to  this  question  will  be 
manifest  on  observing  that  there  comes  into  play  a  cause 
which  gradually  diminishes  the  forces  falling  on  each  new 
layer.  For  the  transverse  sectional  area  is  step  by  step 
increased  ;  and  an  increase  of  the  area  over  which  the  weight 
borne  is  distributed,  implies  a  relatively  smaller  pressure 
upon  each  part  of  it.  Further,  as  the  transverse  dimensions 
of  the  bone  increase,  the  materials  composing  its  convex  and 
concave  layers,  becoming  further  from  the  neutral  axis, 
become  better  placed  for  resisting  the  strains  to  be  borne. 


THE    INNER   TISSUES   OF   ANIMALS.  345 

Bo  that  both  by  the  increased  quantity  of  dense  matter  and 
by  its  mechanically  more- advantageous  position,  the  bendings 
of  the  bone  are  progressively  decreased.  But  as  they  are 
decreased,  each  new  layer  formed  on  the  concave  surface,  has 
its  substance  and  its  capillaries  less  compressed;  and  the 
resulting  growth  and  induration  are  rendered  less  rapid. 
Evidently,  then,  the  additions,  slowly  diminishing,  will 
eventually  cease  ;  and  this  will  happen  when  the  bone  no 
longer  bends.  That  is  to  say,  the  thickening  of  the  bone  will 
i-each  its  limit  when  there  is  equilibrium  between  the  inci- 
dent forces  and  the  forces  which  resist  them.  Here,  indeed, 
we  may  trace  with  great  clearness  the  process  of  direct 
equilibration — may  see  how  an  unusual  force,  falling  on  the 
moving  equilibrium  of  an  organism  and  not  overthrowing  it, 
goes  on  working  modifications  until  the  re-action  balances 
the  action. 

That,  however,  which  now  chiefly  concerns  us,  is  to  note 
how  this  marked  adaptation  supports  the  general  argument. 
Unquestionably  bone  is  in  this  case  formed  under  the  influ- 
ence of  mechanical  stress,  and  formed  just  where  it  most 
effectually  meets  the  stress.  This  result,  not  otherwise 
explained,  is  explained  by  the  h}rpothesis  above  set  forth. 
And  when  we  see  that  this  special  deposit  of  bone  is  ac- 
counted for  by  actions  like  those  to  which  bone-formation  in 
general  is  ascribed,  the  probability  that  these  are  the  actions 
at  work  becomes  very  great. 

Of  course  it  is  not  alleged  that  osseous  structures  arise  in 
this  way  alone.  The  bones  of  the  skull  and  various  dermal 
bones  cannot  be  thus  interpreted.  Here  the  natural  selec- 
tion of  favourable  variations  appears  the  only  assignable 
cause — the  equilibration  is  indirect.  We  know  that  ossific 
deposits  now  and  then  occur  in  tissues  where  they  are  not 
usually  found ;  and  such  deposits,  originally  abnormal,  it' 
they  occurred  in  places  where  advantages  arose  from  them, 
might  readily  be  established  and  increased  by  survival  of  the 
fittest.  Especially  might  we  expect  this  to  happen  when  a 


346  niYsioi.oon  AL  DEVELOPMENT  . 

constitutional  tendency  to  form  bone  had  been  established  by 
actions  of  the  kind  described  ;  for  it  is  a  familiar  fact  that 
differentiated  types  of  tissue,  having  once  become  elements 
of  an  organism,  are  apt  occasionally  to  arise  in  unusual 
places,  and  there  to  repeat  all  their  peculiar  histological  cha- 
racters. And  this  may  possibly  be  the  reason  why  the  bones 
of  the  skull,  though  not  exposed  to  forces  such  as  those  which 
produce,  in  other  bones,  dense  outer  layers  including  less 
dense  interiors,  nevertheless  repeat  this  general  trait  of  bony 
structure.  While,  however,  it  is  beyond  doubt  that  some 
bones  are  not  due  to  the  direct  influence  of  mechanical  stress, 
we  may,  I  think,  conclude  that  mechanical  stress  initiates 
bone-formation. 

§  302.  What  is  the  origin  of  nerve  ?  In  what  way  do  its 
properties  stand  related  to  the  properties  of  that  protoplasm 
whence  the  tissues  in  general  arise  ?  and  in  what  way  is  it 
differentiated  from  protoplasm  simultaneously  with  the  other 
tissues  ?  These  are  profoundly  interesting  questions ;  but 
questions  to  which  positive  answers  cannot  be  expected. 
All  that  can  be  done  is  to  indicate  answers  which  seem 
feasible. 

That  the  property  specially  displayed  by  nerve,  is  a  pro 
perty  which  protoplasm  possesses  in  a  lower  degree,  is  mani 
fest.     The  sarcode  of  a  Rhizopod  and  the  substance  of  an 
unimpregnated  ovum,  exhibit  movements  that  imply  a  propa- 
gation of  stimulus  from  one  part  of  the  mass  to  another  ;  and 
through    the   nerveless   body   of   a    polype,    we   see    slowly 
travelling  and  spreading  a  contraction  set  up  by  touching  a 
tentacle — a  contraction  which  implies  the  passage  from  part  to 
part  of  some  stimulus  causing  the  contraction.  We 

have  not  far  to  seek  for  a  probable  origin  of  this  phenomenon. 
There  is  good  reason  for  ascribing  it  to  the  extreme  insta- 
bility of  the  organic  colloids  of  which  protoplasm  consists. 
These,  in  common  with  colloids  in  general,  assume  different 
isomcric  forms  with  great  facility  ;  and  they  display  not 


THE    INNER   TISSUES   OF   ANIMALS.  347 

simply  isomerism  but  polymerism.  Further,  this  readiness  to 
undergo  molecular  re-arrangement,  habitually  shows  itself  in 
colloids  by  the  rapid  propagation  of  the  re-arrangement 
from  part  to  part.  As  Prof.  Graham  has  shown,  matter  in 
this  state  often  "  pectizes  "  almost  instantaneously — a  touch 
will  transform  an  entire  mass.  That  is  to  say,  the  change  of 
molecular  state  once  set  up  at  one  end,  spreads  to  the  other 
end— there  is  a  progress  of  a  stimulus  to  change  ;  and  this  is 
what  we  see  in  a  nerve.  So  much  being  understood,  let  us 
re-state  the  case  more  completely. 

Molecular  change,  implying  as  it  does  motion  of  molecules, 
communicates  motion  to  adjacent  molecules ;  be  they  of  the 
same  kind  or  of  a  different  kind.  If  the  adjacent  molecules, 
either  of  the  same  kind  or  of  a  different  kind,  be  stable  in 
composition,  a  temporary  increase  of  oscillation  in  them  as 
wholes,  or  in  their  parts,  may  be  the  only  result ;  but  if  they 
are  unstable  there  are  apt  to  arise  changes  of  arrangement 
among  them,  or  among  their  parts,  of  more  or  less  permanent 
kinds.  Especially  is  this  so  with  the  complex  molecules 
which  form  colloidal  matter,  and  with  the  organic  colloids 
above  all.  Hence  it  is  to  be  inferred  that  a  molecular  dis- 
turbance in  any  part  of  a  living  animal,  set  up  by  either  an 
external  or  internal  agency,  will  almost  certainly  disturb  and 
change  some  of  the  surrounding  colloids  not  originally  im- 
plicated— will  diffuse  a  wave  of  change  towards  other  parts 
of  the  organism  :  a  wave  which  will,  in  the  absence  of  per- 
fect homogeneity,  travel  further  in  some  directions  than  in 
others.  Let  us  ask  next  what  will  determine  the 

differences  of  distance  travelled  in  different  directions.  Ob- 
viously any  molecular  agitation  spreading  from  a  centre,  will 
go  furthest  along  routes  that  offer  least  resistance.  What  routes 
will  these  be  ?  Those  along  which  there  lie  most  molecules 
tfiat  are  easily  changed  by  the  diffused  molecular  motion,  and 
which  yet  do  not  take  up  much  molecular  motion  in  assuming 
their  new  states.  Molecules  which  are  tolerably  stable  will 
cot  readily  propagate  the  agitation  ;  for  they  will  absorb  it 


348  PHYSIOLOGICAL   DEVELOPMENT. 

in  the  increase  of  their  own  oscillations,  instead  of  passing  it 
on.  Molecules  which  are  unstable  but  which,  in  assuming 
isomeric  forms,  absorb  motion,  will  not  readily  propagate  it ; 
since  it  will  disappear  in  working  the  changes  in  them.  But- 
unstable  molecules  which,  in  being  isomerically  transformed, 
do  not  absorb  motion,  and  still  more  those  which,  in  being 
so  transformed,  give  out  motion,  will  readily  propagate  any 
molecular  agitation  ;  since  they  will  pass  on  the  impulse  either 
undiminished,  or  increased,  to  adjacent  molecules.  If 

then  we  assume,  as  we  are  not  only  warranted  in  doing  but 
are  obliged  to  do,  that  protoplasm  contains  two  or  more 
colloids,  either  mingled  or  feebly  combined  (since  it  cannot 
consist  of  simple  albumen  or  fibrin  or  casein,  or  any  allied 
proximate  principle)  ;  it  may  be  concluded  that  any  mole- 
cular agitation  set  up  by  what  we  call  a  stimulus,  will  diffuse 
itself  further  along  some  lines  than  along  others,  if  the  com 
ponents  of  the  protoplasm  are  not  quite  homogeneously  dis 
persed,  and  if  some  of  them  are  isomerically  transformed 
more  easily,  or  with  less  expenditure  of  motion,  than 
others ;  and  it  will  especially  travel  along  spaces  occupied 
chiefly  by  those  molecules  which  give  out  molecular  mo- 
tion during  their  metamorphoses,  if  there  should  be  any 
such.  But  now  let  us  ask  what  structural  effects 

will  be  wrought  along  a  tract  traversed  by  this  wave  of 
molecular  disturbance.  As  is  shown  by  those  transforma- 
tions that  so  rapidly  propagate  themselves  through  colloids, 
molecules  that  have  undergone  a  certain  change  of  form, 
are  apt  to  communicate  a  like  change  of  form  to  ad- 
jacent molecules  of  the  same  kind — the  impact  of  each 
overthrow  is  passed  on  and  produces  another  overthrow. 
Probably  the  proneness  towards  isochronism  of  molecular 
movements  necessitates  this.  If  any  molecule  has  had 
its  components  re-arranged,  and  their  oscillations  conse- 
quently altered,  there  result  movements  not  concordant  with 
the  movements  in  adjacent  untransformed  molecules,  but 
which,  impressing  themselves  on  the  parts  of  such  uutrans- 


THE    IXXER   TISSUES    OF    ANIMALS.  349 


formed  molecules,  tend  to  generate  in  them  concordant  move- 
ments —  tend,  that  is,  to  produce  the  re-arrangements  involved 
by  these  concordant  movements.  Is  this  action  limited  to 
strictly  isomeric  substances  ?  or  may  it  extend  to  substances 
that  are  closely  allied?  If  along  with  the  molecules  of  a 
compound  colloid  there  are  mingled  those  of  some  kindred 
colloid  ;  or  if  with  the  molecules  of  this  compound  colloid 
there  are  mingled  the  components  out  of  which  other  such 
molecules  may  be  formed  ;  then  there  arises  the  question  — 
does  the  same  influence  which  tends  to  propagate  the  iso 
meric  transformations,  tend  also  to  form  new  molecules  of 
the  same  kind  out  of  the  adjacent  components  ?  There  is 
reason  to  suspect  that  it  does.  Already  when  treating  of  the 
nutrition  of  parts  (§  64),  it  was  pointed  out  that  we  are  obliged 
to  recognize  a  power  possessed  by  each  tissue  to  build  up,  out 
of  the  materials  brought  to  it,  molecules  of  the  same  type  as 
those  of  which  it  is  formed.  This  building  up  of  like  mole- 
cules seems  explicable  as  caused  by  the  tendency  of  the 
new  components  which  the  blood  supplies,  to  acquire  move- 
ments isochronous  with  those  of  the  like  components  in  the 
tissue  ;  which  they  can  do  only  by  uniting  into  like  com- 
pound molecules.  Necessarily  they  must  gravitate  towards  a 
state  of  equilibrium  ;  such  state  of  equilibrium  —  moving 
equilibrium  of  course  —  must  be  one  in  which  they  oscillate 
in  the  same  times  with  neighbouring  molecules  ;  and  so 
to  oscillate  they  must  fall  into  groups  identical  with  the 
groups  around  them.  If  this  be  a  general  principle  of 
tissue-  growth  and  repair,  we  may  conclude  that  it  will  apply 
in  the  case  before  us.  A  wave  of  molecular  disturbance 
passing  along  a  tract  of  mingled  colloids  closely  allied  in  com- 
position, and  isomerically  transforming  the  molecules  of  one 
of  them,  will  be  apt  at  the  same  time  to  form  some  new  mole- 
cules of  the  same  type,  at  any  place  where  there  exist  the 
proximate  components,  either  uncombined  or  feebly  combined 
in  some  not  very  different  way.  And  this  will  be  most  likely 
to  occur  where  the  molecules  of  the  colloid  that  are  under- 


330  PHYSIOLOGICAL   DEVELOPMENT. 

going  the  isoraeric  change,  predominate,  but  have  scattered 
through  them  the  other  molecules  out  of  which  they  may  be 
formed,  either  by  composition  or  modification.  That  is  to 
say,  a  wave  of  molecular  disturbance  diffused  from  a  centre, 
and  travelling  furthest  along  a  line  where  lie  most  molecules 
that  can  be  isomerically  transformed  with  facility,  will  be 
likely  at  the  same  time  to  further  differentiate  this  line,  and 
make  it  more  characterized  than  before  by  the  easy-trans- 
formabilitv  of  its  molecules.  One  additional  step, 

and  the  interpretation  is  reached.  Analogy  shows  it  to  be 
not  improbable  that  these  organic  colloids,  isomerically  trans- 
formed by  slight  molecular  impact  or  increase  of  molecular 
motion,  will  some  of  them  resume  their  previous  molecular 
structures  after  the  disturbance  has  passed.  We  know  that 
what  are  stable  molecular  arrangements  under  one  degree  of 
molecular  agitation,  are  not  stable  under  another  degree ;  and 
there  is  evidence  that  re-arrangements  of  an  inconspicuous 
kind  are  occasionally  brought  about  by  very  slight  changes 
of  molecular  agitation.  Water  supplies  a  case.  Prof. 
Graham  infers  that  water  undergoes  a  molecular  re-arrange- 
ment at  about  32°— that  ice  has  a  colloid  form  as  well  as  a 
crystalloid  form,  dependent  on  temperature.  Send  through 
it  an  extra  wave  of  the  molecular  agitation  we  call  heat,  and 
its  molecules  aggregate  in  one  way.  Let  the  wave  die  away, 
and  its  molecules  resume  their  previous  mode  of  aggregation. 
And  obviously  such  transformations  may  be  repeated  back- 
wards and  forwards  within  narrow  limits  of  temperature. 
Now  among  the  extremely  unstable  organic  colloids,  such  a 
phenomenon  is  far  more  likely  to  happen.  Suppose,  then,  that 
the  nerve-colloid  is  one  of  which  the  molecules  are  changed  in 
form  by  a  passing  wave  of  extra  agitation,  but  resume  their 
previous  form  when  the  wave  has  passed  :  the  previous  form 
being  the  most  stable  under  the  conditions  which  then  recur. 
What  follows  ?  It  follows  that  these  molecules  will  be  ready 
again  to  undergo  isomerio  transformation  when  there  again 
occurs  the  stimulus ;  will,  as  before,  propagate  the  transform  a- 


•Ulli    IXXEK.   TISSUES    OF    ANIMALS.  351 

lion  most  along  the  tract  where  they  arc  most  abundant;  will, 
as  before,  simultaneously  tend  to  form  new  molecules  of  their 
own  type  ;  will,  as  before,  make  the  line  along  which  they  lie 
one  of  easier  transfer  for  the  molecular  agitation.  Every 
repetition,  will  help  to  increase,  to  integrate,  to  define  more 
completely,  the  course  of-the  escaping  molecular  motion — 
extending  its  remoter  part  while  it  makes  its  nearer  part 
more  permeable — will  help,  that  is,  to  form  a  line  of  discharge, 
a  line  for  conducting  impressions,  a  nerve. 

Such  seems  to  me  a  not  unfair  series  of  deductions  from 
the  known  habitudes  of  colloids  in  general  and  the  organic 
colloids  in  particular.  And  I  think  that  the  implied  nature 
and  properties  of  nerve,  correspond  betler  with  the  observed 
phenomena  than  do  the  nature  and  properties  implied  by 
other  hypotheses.  Of  course  the  speculation  as  it  here  stands 
is  but  tentative,  and  leaves  much  unexplained.  It  gives  no 
obvious  reply  to  the  questions — what  causes  the  formation  of 
nerves  along  some  lines  rather  than  others  ?  what  determines 
their  appropriate  connexions  ? — questions,  however,  to  which, 
when  we  come  to  deal  with  physiological  integration,  we  may 
find  not  unsatisfactory  answers.  Moreover  it  says  nothing 
about  the  genesis  of  ganglia.  A  ganglion,  it  is  clear,  must 
consist  of  a  colloidal  matter  equally  unstable,  or  still  more 
unstable,  which,  when  disturbed,  falls  into  some  different 
molecular  arrangement,  perhaps  chemically  simpler,  and  gives 
out  in  so  doing  a  large  amount  of  molecular  motion — serves 
as  a  reservoir  of  molecular  motion  which  may  be  suddenly 
discharged  along  an  efferent  nerve  or  nerves,  when  excite- 
ment of  an  afferent  nerve  has  disengaged  it.  How  such 
a  structure  as  this  results,  the  hypothesis  does  not  show 
But  admitting  these  shortcomings  it  may  still  be  held  that 
we  are,  in  the  way  pointed  out,  enabled  to  form  an  idea  of 
the  actions  by  which  nervous  tissue  is  differentiated. 

§  303.  A  speculation  akin  to,  and  continuous  with,  the  last, 
is  suggested  by  an  inquiry  into  the  origin  of  muscular  tissue 


3f>2  PHYSIOLOGICAL    DEVELOPMENT. 

Contractility  as  well  as  irritability  is  a  property  of  protoplasm 
or  sarcode ;  and,  as  before  suggested  (§  22),  is  not  improbably 
due  to  isomeric  change  in  one  of  its  component  colloids.  It 
is  a  feasible  supposition  that  of  the  several  isomeric  changes 
simultaneously  set  up  among  these  component  colloids,  some 
may  be  accompanied  by  decided  change  of  bulk  and  some  not. 
Clearly  the  isomeric  change  undergone  by  the  colloid  which 
we  suppose  to  form  nerve,  must  be  one  not  accompanied  by 
appreciable  change  of  bulk ;  since  change  of  bulk  implies 
"  internal  work,"  as  physicists  term  it,  and  therefore  ex- 
penditure of  force.  Conversely,  the  colloid  out  of  which 
muscle  originates,  may  be  one  that  readily  passes  into  an  iso- 
meric state  in  which  it  occupies  less  space :  the  molecular 
disturbance  causing  this  contraction  being  communicated  to 
it  from  adjacent  portions  of  nerve- substance  that  are  mole- 
cularly  disturbed ;  or  being  otherwise  communicated  to  it  by 
direct  mechanical  or  chemical  stimuli ;  as  happens  where 
nerves  do  not  exist,  or  where  their  influence  has  been  cut 
off.  This  interpretation  seems,  indeed,  to  be  directly  at 
variance  with,  the  fact  that  muscle  does  not  diminish  in  bulk 
during  contraction  but  merely  changes  its  shape.  That  which 
we  see  take  place  with  the  muscle  as  a  whole,  is  said  also  to 
take  place  with  each  fibre — while  it  shortens  it  also  broadens. 
There  is,  however,  a  possible  solution  of  this  difficulty.  A 
contracting  colloid  yields  up  its  water ;  and  the  contracted 
colloid  plus  the  free  water,  may  have  the  same  bulk  as  before 
though  the  colloid  has  less.  If  it  be  replied  that  in  this 
ca&e  the  water  should  become  visible  between  the  substance 
of  the  fibre  and  its  sarcolemma  or  sheath,  it  may  be  rejoined 
that  this  is  not  necessary — it  may  be  deposited  interstitially. 
Possibly  the  striated  structure  is  one  that  facilitates  its 
exudation  and  subsequent  re-absorption  ;  and  to  this  may  be 
due  the  superiority  of  striated  muscle  in  rapidity  of  contrac- 
tion. Granting  the  speculative  character  of  this 
interpretation,  let  us  see  how  far  it  agrees  with  the  facts.  If 
the  actions  are  as  here  supposed,  the  contracted  or  more  inte- 


THE    INXEK   TISSUES   OF   AMMAL8.  353 

grated  state  of  the  muscular  colloid  will  be  that  which  it 
tends  continually  to  assume — that  into  which  it  has  an  in- 
creasing aptitude  to  pass  when  artificial  paralysis  has  been 
produced,  as  shown  by  Dr.  Norris — that  into  which  it  lapses 
completely  in  rigor  mortis.  The  sensible  motion  generated 
by  the  contraction  can  arise  only  from  the  transformation 
of  insensible  motion.  This  insensible  motion  suddenly 
yielded  up  by  a  contracting  mass,  implies  the  fall  of  its  com- 
ponent molecules  into  more  stable  arrangements.  And  there 
can  be  no  such  fall  unless  the  previous  arrangement  is  un- 
stable. From  this  point  of  view,  too,  it  is  pos- 
sible to  see  how  the  hydro-carbons  and  oxy-hydro-carbons 
consumed  in  muscular  action,  may  produce  their  effects.  It 
was  said,  when  exposing  The  Data  of  Biology,  that  non-nitro- 
genous substance  might  evolve  heat  only  when  transformed 
in  the  circulating  fluids,  "  but  partly  heat,  and  partly  another 
force,  when  transformed  in  some  active  tissue  that  has  ab- 
sorbed it:  just  as  coal,  though  producing  little  else  but  heat 
as  ordinarily  burnt,  has  its  heat  partially  transformed  into 
mechanical  motion  if  burnt  in  a  steam-engine  furnace " 
(§  18)  ;  and  recent  inquiries  make  it  clear  that  some  such 
relation  exists.*  Here  a  feasible  modus  operandi  becomes 
manifest.  For  these  non-nitrogenous  elements  of  food  when 
consumed  in  the  tissues,  give  out  large  amounts  of  molecular 
motion.  They  do  this  in  presence  of  the  muscular  colloids 
that  have  lost  molecular  motion  during  their  fall  in  the  stable 
or  contracted  state.  And  from  the  molecular  motion  they 
give  out,  may  be  restored  the  molecular  motion  lost  by 
the  contracted  colloids  :  these  contracted  colloids  may 
so  have  their  molecules  raised  to  that  unstable  state  from 
which,  again  falling,  they  can  again  generate  mechanical 
motion. 

*  See  account  of  experiments  made  by  Profs.  Fick  and  Wislicenus,  trans- 
lated by  Prof.  Wanklyn  in  the  Phil.  Mag.  for  May  or  June,  1866.  Soe 
also  an  article  by  Prof.  Frankland  iu  the  September  number  of  the  saini 
journal. 


354  PHYSIOLOGICAL    DEVELOPMENT. 

This  conception  of  the  nature  and  mode  of  action  of  muscle, 
while  it  is  suggested  by  known  properties  of  colloidal  matter 
and  conforms  to  the  recent  conclusions  of  organic  chemistry 
and  molecular  physics,  establishes  a  comprehensible  relation 
between  the  vital  actions  of  the  lower  and  the  higher  animals. 
If  we  contemplate  the  movements  of  cilia,  of  a  Hhizopod's 
pseudo-podia,  of  a  Polype's  body,  or  of  the  long  pendant  ten- 
tacles of  a  Medusa,  we  shall  see  great  congruity  between 
them  and  this  hypothesis.  Bearing  in  mind  that  the  con- 
tractile substance  of  developed  muscle  is  affected  not  by 
nervous  influence  only,  but,  where  nervous  influence  is 
destroyed,  is  made  to  contract  by  mechanical  disturbance  and 
chemical  action,  we  may  infer  that  it  does  not  differ  intrin- 
sically from  the  primordial  contractile  substance,  which,  in 
the  lowest  animals,  changes  its  bulk  under  other  stimuli  than 
the  nervous.  "We  shall  see  significance  in  the  fact  ascer- 
tained by  Dr.  Ransom,  that  various  agents  which  excite 
and  arrest  nervo-muscular  movements  in  developed  animals, 
excite  and  arrest  the  protoplasmic  movements  in  ova.  We 
shall  understand  how  tissues  not  yet  differentiated  into  muscle 
and  nerve,  have  this  joint  irritability  and  contractility ;  how 
muscle  and  nerve  may  arise  bv  the  segregation  of  their 
mingled  colloids,  the  one  of  which,  not  appreciably  altering 
its  bulk  during  isomeric  change,  readily  propagates  molecular 
disturbance,  while  the  other,  contracting  when  isomerically 
changed,  less  readily  passes  on  the  molecular  disturbance ; 
and  how  by  this  differentiation  and  integration  of  the  con- 
ducting and  the  contracting  colloids,  the  one  ramifying 
through  the  other,  it  becomes  possible  for  a  whole  mass  to 
contract  suddenly,  instead  of  contracting  gradually,  as  it  does 
when  undifferentiated. 

The  question  remaining  to  be  asked  is — What  causes  tho 
specialization  of  contractile  substance  ? — What  causes  the 
growth  of  colloid  masses  which  monopolize  this  contractility, 
and  leave  kindred  colloids  to  monopolize  other  properties  ? 
Has  natural  selection  gradually  localized  and  increased 


T11E    INKER  TISSUES   OF    ANIMALS.  355 

the  primordial  muscular  substance  ?  or  has  the  frequent  recur 
mice  of  irritations  and  consequent  contractions  at  particular 
parts  done  it  ?  We  have,  I  think,  reason  to  conclude  that 
direct  equilibration  rather  than  indirect  equilibration  has  been 
chiefly  operative.  The  reasoning  that  was  used  in  the  case 
of  nerve  applies  equally  in  the  case  of  muscle.  A  portion  of 
uridifFerentiated  tissue  containing  a  predominance  of  the  colloid 
that  contracts  in  changing,  will,  during  each  change,  tend 
to  form  new  molecules  of  its  own  type  from  the  other  colloids 
diffused  through  it :  the  tendency  of  these  entangled  colloids 
to  fall  into  unity  with  those  around  them,  will  be  aided  by 
every  shock  of  isomeric  transformation.  Hence,  repeated 
contractions  will  further  the  growth  of  the  contracting  mass, 
and  advance  its  differentiation  and.  integration.  If, 

too,  we  remember  that  the  muscular  colloid  is  made  to 
contract  by  mechanical  disturbance,  and  that  among  me- 
chanical disturbances  one  which  will  most  readily  affect  it 
simultaneously  throughout  its  mass  is  caused  by  stretching, 
we  shall  be  considerably  helped  towards  understanding  how 
the  contractile  tissues  are  developed.  If  extension  of  a  mus- 
cular colloid  previously  at  rest,  produces  in  it  that  molecular 
disturbance  that  leads  to  isomeric  change  and  decrease  of 
bulk,  then  there  is  no  difficulty  in  explaining  the  movements 
of  cilia.  The  formation  of  a  contractile  layer  in  the  vascular 
system  becomes  comprehensible :  each  dilatation  of  a  blood- 
vessel caused  by  a  gush  of  blood,  will  be  followed  by  a  con- 
striction ;  the  heart  will  pulsate  violently  in  proportion  as 
it  is  violently  distended ;  arteries  will  develop  in  power  as 
the  stress  upon  them  becomes  greater.  And  we  shall  simi- 
larly have  an  explanation  of  the  increased  muscularity  of 
the  alimentary  canal  that  is  brought  about  by  increased 
distension  of  it. 

That  the  production  of  contractile  tissue  in  certain  localities, 
is  due  to  the  more  frequent  excitement  in  those  localities 
of  the  contractility  possessed  by  undifferentiated  tissue  in 
general,  is  a  view  harmonizing  with  facts  which  the  diffe- 


356  PHYSIOLOGICAL   DEVELOPMENT. 

rentiated  contractile  tissues  exhibit.  These  are  the  rela- 
tions between  muscular  exercise,  muscular  power,  and  mus- 
cular structure  ;  and  it  is  the  more  needful  for  us  here  to 
notice  them  because  of  certain  anomalies  they  present, 
which,  at  first  sight,  seem  inconsistent  with  the  belief  that 
the  functionally- determined  modifications  of  muscle  are  in- 
heritable. 

Muscles  disagree  greatly  in  their  tints — all  gradations 
between  white  and  deep  red  being  observable.  Contrasts 
are  visible  between  the  muscles  of  different  animals,  be- 
tween the  muscles  of  the  same  animal  at  different  ages,  and 
between  different  muscles  of  the  same  animal  at  the  same 
age.  We  will  glance  at  the  facts  under  these  heads  :  noting 
under  each  of  them  the  connexion  which  here  chiefly  con- 
cerns us — that  between  the  activity  of  muscle  and  its  depth 
of  colour.  The  cold-blooded  Vertebrata  are,  taken 

as  a  group,  distinguished  from  the  warm-blooded  by  the 
whiteness  of  their  flesh  ;  and  they  are  also  distinguished  by 
their  comparative  inertness.  Though  a  fish  or  a  reptile  can 
exert  considerable  force  for  a  short  time,  it  is  not  capable  of 
prolonged  exertion.  Birds  and  mammals  show  greater  en- 
durance along  with  darker- coloured  muscles.  If  among  birds 
themselves  or  mammals  themselves  we  make  comparisons,  we 
meet  with  kindred  contrasts — especially  between  wild  and 
domestic  creatures  of  allied  kinds.  Barn-door  fowls  are 
lighter-fleshed  than  most  untamed  gallinaceous  birds ;  and 
among  these  last  the  pheasant,  moving  about  but  little,  id 
lighter-fleshed  than  the  partridge  and  the  grouse  which  are 
more  nomadic.  The  muscles  of  the  sheep  are  not  on  the 
average  so  dark  as  those  of  the  deer  ;  and  it  is  said  that  the 
flesh  of  the  wild-boar  is  darker  than  that  of  the  pig 
Perhaps,  however,  the  contrast  between  the  hare  and  the 
rabbit  affords,  among  familiar  animals,  the  best  example  of 
the  alleged  relation  :  the  dark- fleshed  hare  having  no  retreat 
and  making  wide  excursions,  while  the  white-fleshed  rabbit, 
passing  a  great  part  of  its  time  in  its  burrow,  rarely  wanders 


THE    INMElt    TISSUES    OF    ANIMALS.  357 

far  from    home.  The   parallel    contrast    between 

young  and  old  animals  has  a  parallel  meaning.  Veal  is 
much  whiter  than  beef,  and  lamb  is  of  lighter  colour  than 
mutton.  Though  at  first  sight  these  facts  may  not  seem 
to  furnish  confirmatory  evidence,  since  lambs  in  their  play 
appear  to  expend  more  muscular  force  than  their  sedate 
dams ;  yet  the  meaning  of  the  contrast  is  really  as  alleged. 
For  in  consequence  of  the  law  that  the  strains  which  animals 
have  to  overcome,  increase  as  the  cubes  of  the  dimensions, 
while  their  powers  of  overcoming  them  increase  only  as  the 
squares  (§  46),  the  movements  of  an  adult  animal  cost  very 
much  more  in  muscular  effort  than  do  those  of  a  young 
animal :  the  result  being  that  the  sheep  and  the  cow  exercise 
their  muscles  more  vigorously  in  their  quiet  movements,  than 
the  lamb  and  the  calf  in  their  lively  movements.  It  may  be 
added  as  significant,  that  the  domestic  animal  in  which  no 
very  marked  darkening  of  the  flesh  takes  place  along  with 
increasing  age,  namely  the  pig,  is  one  which,  ordinarily  kept 
in  a  sty,  leads  so  quiescent  a  life  that  the  assigned  cause  of 
darkening  does  not  come  into  action.  But  perhaps 

the  most  conclusive  evidences  are  the  contrasts  that  exist 
between  the  active  and  inactive  muscles  of  the  same  animal. 
I3et\veen  the  leg-muscles  of  fowls  and  their  pectoral  muscles, 
the  difference  of  colour  is  familiar  ;  and  we  know  that  fowls 
exercise  their  leg-muscles  much  more  than  the  muscles  which 
move  their  wings.  Similarly  in  the  turkey,  in  the  guinea 
fowl,  in  the  pheasant.  And  then,  adding  much  to  the  force  of 
this  evidence,  we  see  that  in  partridges  and  grouse,  which 
belong  to  the  same  order  as  our  domestic  fowls,  but  use  their 
wings  as  habitually  as  their  legs,  little  or  no  difference  is 
visible  between  the  colours  of  these  two  groups  of  muscles. 
Special  contrasts  like  these  do  not,  however,  exhaust  the 
proofs  ;  for  there  is  a  still  more  significent  general  contrast. 
The  muscle  of  the  heart,  which  is  the  most  active  of  all 
muscles,  is  the  darkest  of  all  muscles. 

The  connection  of  phenomena  thus  shown  in  so  many  waj78, 


358  PHYSIOLOGICAL    DEVELOPMENT. 

implies  that  the  bulk  of  a  muscle  is  by  no  means  the  sole 
measure  of  the  quantity  of  force  it  can  evolve.  It  would  seem 
that,  other  things  equal,  the  depth  of  colour  varies  with  the 
constancy  of  action  ;  while,  other  things  equal,  the  bulk  varies 
with  the  amount  of  force  that  has  to  be  put  forth  upon  oc- 
casion. These  of  course  are  approximate  relations.  More 
correctly  we  may  say  that  the  actions  of  pale  muscles  are 
either  relatively  feeble  though  frequent  (as  in  the  massive 
flanks  of  a  fish),  or  relatively  infrequent  though  strong  (as  in 
the  pectoral  muscles  of  a  common  fowl)  ;  while  the  actions  of 
dark  muscles  are  both  frequent  and  strong.  Some  such  dif- 
ferentiation may  be  anticipated  by  inference  from  the  respec- 
tive physiological  requirements.  A  muscle  which  has  upon 
occasion  to  evolve  considerable  force,  but  which  has  thereafter 
a  long  period  of  rest  during  which  repair  may  restore  it  to 
efficiency,  requires  neither  a  large  reserve  of  the  contrac- 
tile substance  that  is  in  some  way  deteriorated  by  action, 
nor  highly-developed  appliances  for  bringing  it  nutri- 
tive materials  and  removing  effete  products.  Where,  con- 
trariwise, an  exerted  muscle  that  has  undergone  much 
molecular  change  in  evolving  mechanical  force,  has  soon  again 
to  evolve  much  mechanical  force,  and  so  on  continually  ;  it 
is  clear  that  either  the  quantity  of  contractile  substance 
present  must  be  great,  or  the  apparatus  for  nutrition  and 
depuration  must  be  very  efficient,  or  both.  Hence  we  may 
look  for  marked  unlikenesses  of  minute  structure  between 
muscles  that  are  markedly  contrasted  in  activity.  And  we  may 
suspect  that,  these  conspicuous  contrasts  of  colour  between 
active  and  inactive  muscles,  are  due  to  these  implied  diffe- 
rences of  minute  structure — partly  differences  between  the 
numbers  of  blood-vessels  and  partly  differences  between  the 
quantities  of  sarcous  matter. 

Here,  then,  we  have  a  key  to  the  apparent  anomaly  above 
hinted  at — the  maintenance  of  bulk  by  certain  muscles  which 
have  been  rendered  comparatively  inactive  by  changed  habits 
of  life.  That  the  pectoral  muscles  of  those  domestic  birds 


THE    INNER   TISSUES   OF    ANIMALS.  359 

which  fly  but  little,  have  not  dwindled  to  any  great  extent, 
has  been  thought  a  fact  at  variance  with  the  conclusion  that 
i'unctioually-produced  adaptations  are  inheritable.  It  has 
been  argued  that  if  parts  which  are  exercised  increase,  not 
only  iu  the  individual  but  in  the  race,  while  parts  which 
become  less  active  decrease  ;  then  a  notable  difference  of  size 
should  exist  between  the  muscles  used  for  flight  in  birds  that 
fly  much,  and  those  in  birds  of  an  allied  kind  that  fly  little. 
But,  as  we  here  see,  this  is  not  the  true  implication.  The 
change  in  such  cases  must  be  chiefly  in  vascularity  and  abun- 
dance of  contractile  substance  ;  and  cannot  be,  to  any  great 
extent,  in  bulk.  For  a  bird  to  fly  at  all,  its  pectoral  muscles, 
bones  of  attachment,  and  all  accompanying  appliances,  must 
be  kept  up  to  a  certain  level  of  power.  If  the  parts  dwindle 
much,  the  creature  will  be  unable  to  lift  itself  from  the 
ground.  Bearing  in  mind  that  the  force  which  a  bird  ex- 
pends to  sustain  itself  in  the  air  during  each  successive  instant 
of  a  short  flight,  is,  other  things  equal,  the  same  as  it  ex- 
pends in  each  successive  instant  of  a  long  flight,  we  shall  see 
that  the  muscles  employed  in.  the  two  cases  must  have  some 
thing  like  equal  intensities  of  contractile  power  ;  and  that  the 
structural  differences  between  them  must  have  relation  mainly 
to  the  lengths  of  time  during  which  they  can  continue  to  re- 
peat contractions  of  like  intensity.  That  is  to  say,  while  the 
power  of  flight  is  retained  at  all,  the  muscles  and  bones  can- 
not greatly  dwindle;  but  the  dwindling,  in  birds  whose  flights 
are  short  or  infrequent  or  both,  will  be  in  the  reserve  stock 
of  the  substance  that  is  incapacitated  by  action,  or  in  the 
appliances  that  keep  the  apparatus  in  repair,  or  in  both. 
Only  where,  as  in  the  struthious  birds,  the  habit  of  flight  is 
lost,  can  we  expect  atrophy  of  all  the  parts  concerned  in 
flight ;  and  here  we  find  it. 

Are  such  differentiations  among  the  muscles  functionally 

produced  ?  or  are  they  produced  by  the  natural  selection  of 

variations  distinguished  as  spontaneous  ?     We  have,  I  think, 

good  grounds  for  concluding  that  they  are  functionally  pro- 

VOL.  IL  \f> 


360  PHYSIOLOGICAL   DEVELOPMENT. 

duced.  We  know  that  in  individual  men  and  animals,  the 
power  of  sustained  action  in  muscles  is  rapidly  adaptable  to 
the  amount  of  sustained  action  required.  We  know  that 
being  "out  of  condition,"  is  usually  less  shown  by  the  inability 
to  put  out  a  violent  effort  than  by  the  inability  to  continue 
making  violent  efforts ;  and  we  know  that  the  result  of  train- 
ing for  prize-fights  and  races,  is  more  shown  in  the  prolonga- 
tion of  energy  than  in  the  intensification  of  energy.  At  the 
same  time,  experience  has  taught  us  that  the  structural  change 
which  accompanies  this  functional  change,  is  not  so  much  a 
change  in  the  bulk  of  the  muscles  as  a  change  in  their  inter- 
nal state  :  instead  of  being  soft  and  flabby  they  become  hard. 
We  have  inductive  proof,  then,  that  exercise  of  a  muscle  causes 
some  interstitial  growth  along  with  the  power  of  more  sus- 
tained action ;  and  there  can  be  no  doubt  that  the  one  is  a 
condition  to  the  other.  What  is  this  interstitial  growth  ? 
There  is  reason  to  suspect  that  it  is  in  part  an.  increased 
deposit  of  the  sarcous  substance  and  in  part  a  development  of 
blood-vessels.  Microscopic  observation  tends  to  confirm  the 
conclusions  before  drawn,  that  repetition  of  contractions  fur- 
thers the  formation  of  the  matter  which  contracts,  and  that 
greater  draughts  of  blood  determine  greater  vascularity. 
And  if  the  contrasts  of  molecular  structure  and  the  contrasts 
of  vascularity,  directly  caused  in  muscles  by  contrasts  in  their 
activities,  are  to  any  degree  inheritable  ;  there  results  an 
explanation  of  those  constitutional  differences  in  the  colours 
and  textures  of  muscles,  which  accompany  constitutional 
differences  in  their  degrees  of  activity. 

It  may  be  added  that  if  we  are  warranted  in  so  ascribing 
the  differentiations  of  muscles  from  one  another  to  direct 
equilibration,  then  we  have  the  more  reason  for  thinking 
that  the  differenti.ation  of  muscles  in  general  from  other 
structures  is  also  due  to  direct  equilibration.  That  unlike- 
nesses  between  parts  of  the  contractile  tissues  having  unlike 
functions,  are  caused  by  the  unlikenesses  of  their  functions, 
renders  it  the  more  probable  that  the  uulikenesses  between 


1HE    INNER   TISSUES   OF  ANIMALS.  36i 

contractile  tissue  and  other  tissues,  have  been  caused  by  ana- 
logous unlikenesses. 

§  304.  These  interpretations,  which  have  already  occupied 
too  large  a  space,  must  here  be  closed.  Of  course  out  of 
phenomena  so  multitudinous  and  varied,  it  has  been  imprac- 
ticable to  deal  with  any  but  the  most  important ;  and  it  has 
been  practicable  to  deal  with  these  only  in  a  general  way. 
Much,  however,  as  remains  to  be  explained,  I  think  the  possi- 
bility of  tracing,  in  so  many  cases,  the  actions  to  which  these 
internal  differentiations  may  rationally  be  ascribed,  makes  it 
likely  that  the  remaining  internal  differentiations  are  due  to 
kindred  actions.  We  find  evidence  that  in  more  cases 
than  seemed  probable,  these  actions  produce  their  effects 
directly  on  the  individual ;  and  that  the  unlikenesses  are 
produced  by  accumulation  of  such  effects  from  generation  to 
generation.  While  for  the  remaining  unlikenesses,  we  have, 
as  an  adequate  cause,  the  indirect  effects  wrought  by  the  sur- 
vival, generation  after  generation,  of  the  individuals  in  which 
favourable  variations  have  occurred — variations  such  as  those 
of  which  human  anatomy  furnishes  endless  instances.  Thus 
accounting  for  so  much,  we  may  not  unreasonably  presume 
that  these  co-operative  processes  of  direct  and  indirect  equili- 
bration will  account  for  what  remains. 

Though  not  strictly  included  under  the  title  of  the  chap- 
ter, there  is  a  subject  on  which  a  few  words  may  here  be 
added,  because  of  the  elucidations  yielded  to  it  by  some 
parts  of  the  chapter.  I  refer  to  the  repair  and  growth  of  the 
differentiated  tissues.  When  treating  inductively  of  that  resto- 
ration which  takes  place  in  worn  organs,  it  was  admitted  that 
little  in  the  way  of  deductive  interpretation  is  apparent — 
nothing  beyond  the  harmony  between  the  facts  and  the 
general  principle  of  segregation  (§  64).  And  it  was  further 
admitted  that  it  is  not  obvious  why,  within  certain  limits,  an 
organ  grows  in  proportion  as  it  is  exercised.  Certain  of  the 
foregoing  considerations,  however,  help  us  towards  a  partial 


362  PHYSIOLOGICAL   DEVELOPMENT. 

rationale  of  these  phenomena.  When  treating  of  the  de» 
veloprnent  of  respiratory  surfaces,  external  or  internal,  afc 
places  where  the  greatest  contrast  exists  between  the  oxy- 
genated plasma  outside  the  vessels  and  the  carbonized  blood 
inside  them,  reference  was  made  to  the  truth  that  the  ex- 
change of  liquids  must,  other  things  equal,  be  rapid  in  pro- 
portion as  the  contrast  between  them  is  great.  Now  this 
truth  holds  generally.  In  every  tissue  the  rate  of  osmotic 
exchange  must  vary  as  this  contrast  varies ;  and  where  the 
contrast  is  produced  by  composition  or  decomposition  going 
forward  in  the  tissue,  the  amount  of  exchange  must  be  pro- 
portionate to  the  amount  of  composition  or  decomposition. 
If  the  blood  is  circulating  through  an  inactive  organ,  there 
is  nothing  to  disturb,  in  any  great  degree,  the  proximate 
equilibrium  between  the  plasma  within  the  blood-vessels  and 
the  plasma  without  them.  But  if  the  tissue  is  functionally 
excited — if  it  is  made  to  yield  up  and  expend  part  of  the  force 
latent  in  its  molecules  or  the  molecules  of  the  oxy-hydro- 
carbons  permeating  it,  its  contained  liquid  necessarily  becomes 
charged  with  molecules  of  another  order — simpler  molecules ; 
and  the  greater  the  amount  of  function  the  more  different 
is  it  made  from  the  liquid  contained  in  the  blood-vessels. 
Hence  the  osmotic  exchange  must  be  most  rapid  where  the 
metamorphosis  of  substance  is  most  rapid — the  materials  for 
consumption  and  for  re-integration  of  tissue,  must  be  supplied 
in  proportion  to  the  demand.  This,  however,  is  not  the  sole 
process  by  which  waste  and  repair  are  equilibrated.  There 
is  the  osmotic  distension  above  pointed  out  as  one  of  the 
causes  of  circulation — a  force  tending  ever  to  thrust  most 
blood  to  the  places  where  there  is  the  greatest  escape  for  it : 
that  is — the  greatest  consumption  of  it.  For  since  in  an  active 
tissue,  the  plasma  passing  out  of  its  capillaries  into  its  sub- 
stance is  continually  yielding  up  its  complex  molecules, 
either  to  be  assimilated  or  to  be  decomposed  ;  and  since  the 
products  of  decomposition,  whether  of  the  nitrogenous  tissue 
or  of  its  contained  hydro- carbons,  are  simpler  than  the 


THE    INNER    TISSUES   OF   ANIMALS.  363 

substances  from  which  they  arise,  and  therefore  have  greater 
molecular  mobility ;  it  follows  that  the  liquid  contained  in 
an  active  tissue  has  a  greater  average  molecular  mobility 
than  the  liquids  elsewhere;  and  therefore  makes  its  way 
through  the  channels  of  excretion  faster  than  elsewhere  :  the 
two  chief  products,  carbonic  acid  and  water,  escaping  with 
especial  facility.  Hence  the  place  becomes  a  place  of  least 
resistance,  through  which  the  distended  walls  of  the  elastic 
vascular  system  tend  continually  to  force  out  an  extra 
quantity  of  plasma.  The  argument  carried  a 

step  further,  yields  us  an  idea  of  the  way  in  which  not  only 
repair  but  also  growth  of  the  exercised  tissue  may  be  caused 
— at  least,  where  this  tissue  is  one  which  evolves  force. 
Assuming  it  to  be  established  that  the  force  generated 
by  muscle  does  not  result  from  the  consumption  of  its  nitro- 
genous substance,  but  from  the  consumption  of  its  contained 
hydro-carbons  and  oxy-hydro-carbons ;  and  inferring  that  a 
large  amount  of  muscular  action  may  be  performed  without 
a  corresponding  loss  of  nitrogenous  substance ;  we  get  a 
clue  to  the  process  of  increase  in  a  specially-exercised 
muscle.  For  if  osmotic  exchange  and  osmotic  distension 
conspire  to  produce  a  more  rapid  passage  of  plasma  out 
of  the  capillaries  into  this  active  tissue  than  into  inactive 
tissues ;  and  if,  of  the  substances  in  this  larger  supply  of 
plasma,  only  the  non-nitrogenous  are  consumed ;  then  there 
must  be  an  accumulation  of  the  nitrogenous  substances.  If 
the  waste  of  the  albuminous  components  of  the  tissue  has 
not  kept  pace  with  the  consumption  of  its  carbonaceous  con- 
tents; then  there  will  exist  in  the  liquid  permeating  it  more 
albuminous  substance  than  is  needed  for  its  repair — there 
will  be  material  for  its  growth.  The  growth  thus  resulting, 
however,  will  be  limited  both  by  the  capacity  of  the  channels 
of  supply  and  by  the  competing  absorption  of  other  active 
tissues.  So  long  as  one  muscle,  or  set  of  muscles,  is 
specially  exercised,  while  the  rest  discharge  but  small 
amounts  of  duty — so  long, '  that  is,  as  the  quantity  of 


364  PHYSIOLOGICAL   DEVELOPMENT. 

tissue-forming  matters  taken  from  the  alimentary  canal  into 
the  blood,  is  not  largely  draughted  off  elsewhere,  this  local 
growth  may  go  on.  But  if  many  other  sets  of  muscles  are 
similarly  active,  the  abstraction  of  tissue- forming  matters  at 
various  places,  will  so  far  diminish  their  abundance  in  the 
blood,  as  to  reduce  the  supply  available  at  any  one  place  for 
growth :  eventually  leaving  sufficient  for  repair  only. 

Though  we  lack  data  for  thus  interpreting  specifically 
the  repair  and  growth  of  other  active  tissues,  yet  we  may  see, 
in  a  general  way,  that  a  parallel  interpretation  holds.  For 
if  any  tissue  that  consumes,  transforms,  excretes,  or  secretes 
matters  that  pass  into  it  from  the  blood,  is  not  formed  of  the 
same  constituents  as  these  matters  it  transforms  or  excretes  ; 
or  if  it  does  not  undergo  waste  proportionate  to  the  quantity 
of  matter  it  transforms  or  excretes  ;  then  it  seems  fairly 
inferable  that  along  with  any  unusual  quantity  of  such 
matters  to  be  transformed  or  excreted,  the  plasma  passing  into 
it  must  bring  a  surplus  of  the  materials  for  its  own  repair 
and  growth. 


CHAPTER  IX. 

PHYSIOLOGICAL    INTEGRATION   IN    ANIMALS. 

§  305.  Physiological  differentiation  and  physiological  inte- 
gration, are  correlatives  that  vary  together.  We  have  but 
to  recollect  the  familiar  parallel  between  the  division  of 
labour  in  a  society  and  the  physiological  division  of  la- 
bour, to  see  that  as  fast  as  the  kinds  of  work  performed  by 
the  component  parts  of  an  organism  become  more  numerous, 
and  as  fast  as  each  part  becomes  more  restricted  to  its  own 
work,  so  fast  must  the  parts  have  their  actions  combined  in 
such  ways  that  no  one  can  go  on  without  the  rest  and  the 
rest  cannot  go  on  without  each  one. 

Here  our  inquiry  must  be,  how  the  relationship  of 
these  two  processes  is  established — what  causes  the  inte- 
gration to  advance  pan  passu  with  the  differentiation. 
Though  it  is  manifest,  d  priori,  that  the  mutual  dependence 
of  functions  must  be  proportionate  to  the  specialization  of 
functions ;  yet  it  remains  to  find  the  mode  in  which  the  in- 
creasing co-ordination  is  determined. 

Already,  among  the  Inductions  of  Biology,  this  relation 
between  differentiation  and  integration  has  been  specified 
and  illustrated  (§  59).  Before  dealing  with  it  deductively, 
a  few  further  examples,  grouped  so  as  to  exhibit  its  several 
uspects,  will  be  advantageous. 

§  306.  If  the  lowly-organized  Planaria  has  its  body 
broken  up  and  its  gullet  detached,  this  will,  for  a  while, 


366  PHYSIOLOGICAL   DEVELOPMENT. 

continue  to  perform  its  function  when  called  upon,  just 
as  though  it  were  in  its  place  :  a  fragment  of  the  creature's 
own  body  placed  in  the  gullet,  will  be  propelled  through  it, 
or  swallowed  by  it.  But,  as  the  seeming  strangeness  of  this 
fact  implies,  we  find  no  such  independent  actions  of  analogous 
parts  in  the  higher  animals.  A  piece  cut  out  of  the 

disc  of  a  Medusa,  continues  with  great  persistence  repeating 
those  rhythmical  contractions  which  we  see  in  the  disc  as 
a  whole ;  and  thus  proves  to  us  that  the  contractile  function 
in  each  portion  of  the  disc,  is  in  great  measure  independent 
But  it  is  not  so  with  the  locomotive  organs  of  more  differen- 
tiated types.  When  separated  from  the  rest,  these  lose  their 
powers  of  movement.  The  only  member  of  a  vertebrate  animal 
which  continues  to  act  after  detachment,  is  the  heart ;  and 
the  heart  has  a  motor  apparatus  complete  within  itself. 

Where  there  is  this  small  dependence  of  each  part  upon 
the  whole,  there  is  but  small  dependence  of  the  whole 
upon  each  part.  The  longer  time  which  it  takes  for  the 
arrest  of  a  function  to  produce  death  in  a  less  differentiated 
animal  than  in  a  more  differentiated  animal,  may  be  illus- 
trated by  the  case  of  respiration.  Suffocation  in  a  man 
speedily  causes  resistance  to  the  passage  of  the  blood  through 
the  capillaries,  followed  by  congestion  and  stoppage  of  the 
heart :  great  disturbance  throughout  the  system  results  in  a 
few  seconds  ;  and  in  a  minute  or  two  all  the  functions  cease. 
But  in  a  frog,  with  its  undeveloped  respiratory  organ,  and  a 
skin  through  which  a  considerable  aeration  of  the  blood  is 
carried  on,  breathing  may  be  suspended  for  a  long  time 
without  injury.  Doubtless  this  difference  is  proximately  due 
to  the  greater  functional  activity  in  the  one  case  than  in  the 
other,  and  the  more  pressing  need  for  discharging  the  pro- 
duced carbonic  acid  ;  but  the  greater  functional  activity  being 
itself  made  possible  by  the  higher  specialization  of  functions, 
this  remains  the  primary  cause  of  the  greater  dependence  of 
the  other  functions  on  respiration,  where  the  respiratory 
apparatus  has  become  highly  specialized.  Ilere, 


PHYSIOLOGICAL   INTEGRATION   IN   ANIMALS.  367 

indeed,  we  see  the  relation,  under  another  aspect.  This  more 
rapid  rhythm  of  the  functions  which  increased  heterogeneity 
of  structure  makes  possible,  is  itself  a  means  of  integrating 
the  functions.  Watch,  when  it  is  running  down,  a  compli- 
cated machine  of  which  the  parts  are  not  accurately  adjusted, 
or  are  so  worn  as  to  be  somewhat  loose.  There  will  be 
observed  certain  irregularities  of  movement  just  before  it 
comes  to  rest — certain  of  the  parts  which  stop  first,  are 
again  made  to  move  a  little  by  the  continued  movement 
of  the  rest,  and  then  become  themselves,  in  turn,  the 
causes  of  renewed  motion  in  other  parts  which  have  ceased 
to  move.  That  is  to  say,  while  the  connected  rhythmical 
changes  of  the  machine  are  quick,  their  actions  and  re- 
actions on  one  another  are  regular — all  the  motions  are  well 
integrated;  but  as  the  velocity  diminishes,  irregularities  arise 
— the  motions  become  somewhat  disintegrated.  Similarly 
with  organic  functions :  increase  of  their  rapidity  involves 
increase  of  a  joint  momentum  which  controls  each  and  co- 
ordinates all.  Thus,  if  we  compare  a  Snake  with  a  Mammal, 
we  see  that  its  functions  are  not  tied  together  so  closely. 
The  Mammal,  and  especially  the  superior  Mammal,  requires 
food  with  considerable  regularity  ;  keeps  up  a  respiration 
that  varies  within  but  moderate  limits  ;  and  has  periods  of 
activity  and  rest  that  alternate  evenly  and  frequently.  But 
the  Snake,  taking  food  at  long  intervals,  may  have  these 
intervals  greatly  extended  without  fatal  results  ;  its  dormant 
and  its  active  states  recur  less  uniformly  ;  and  its  rate  of 
respiration  varies  within  much  wider  limits — now  being 
scarcely  perceptible,  and  now,  as  you  may  prove  by  exciting 
it,  becoming  conspicuous.  So  that  here,  where  the  rhythms 
are  very  slow,  they  are  individually  less  regular,  and  are 
united  into  a  less  regular  compound  rhythm — are  less  in- 
tegrated. 

Perhaps  the  clearest  general  idea  of  the  co-ordination  of 
functions  that  accompanies  their  specialization,  is  obtained  by 
observing  the  slowness  with  which  a  little-differentiated  animal 


368  PHYSIOLOGICAL   DEVELOPMKNT. 

responds  to  a  stimulus  applied  to  one  of  its  parts,  and  the 
rapidity  with  which  such  a  local  stimulus  is  responded  to  by 
a  more-differentiated  animal.  A  Polype  and  a  Polyzoon,  two 
creatures  somewhat  similar  in  their  outward  appearances  but 
very  unlike  in  their  internal  structures,  will  serve  for  the 
comparison.  A  tentacle  of  a  Polype,  when  touched,  slowly 
contracts  ;  and  if  the  touch  has  been  rude,  the  contraction 
presently  extends  to  the  other  tentacles  and  eventually  to  the 
entire  body :  the  stimulus  to  movement  is  gradually  diffused 
throughout  the  organism.  But  if  you  touch  a  tentacle  of  a 
Polyzoon,  or  slightly  disturb  the  water  near  it,  the  whole 
cluster  of  tentacles  is  instantly  withdrawn,  along  with  the 
protruded  part  of  the  creature's  body,  into  its  sheath.  Whence 
arises  this  contrast  ?  The  one  creature  has  no  specialized 
contractile  organs,  or  fibres  for  conveying  impressions.  Tiie 
other  has  definite  muscles  and  nerves.  The  parts  of  the 
little-differentiated  Polype  have  their  functions  so  feebly  co- 
ordinated, that  one  may  be  strongly  affected  for  a  long  time 
before  any  effect  is  felt  by  another  at  a  distance  from  it ;  but 
in  the  more-differentiated  Polyzoon,  various  remote  parts 
instantly  have  changes  propagated  to  them  from  the  affected 
part,  and  by  their  united  actions  thus  set  up,  the  whole 
organism  adjusts  itself  so  as  to  avoid  the  danger. 

These  few  added  illustrations  will  make  the  nature  of  this 
general  relation  sufficiently  clear.  Let  us  now  pass  to  the 
interpretation  of  it. 

§  307.  If  a  Hydra  is  cut  in  two,  the  nutritive  liquids 
diffused  through  its  substance  cannot  escape  rapidly,  since 
there  are  no  open  channels  for  them  ;  and  hence  the  condi- 
tion of  the  parts  at  a  distance  from  the  cut  is  but  little 
affected.  But. where,  as  in  the  more-differentiated  animals, 
the  nutritive  liquid  is  contained  in  vessels  that  have  con- 
'inuous  communications,  cutting  the  body  in  two,  or  cutting 
off  any  considerable  portion  of  it,  is  followed  by  escape  of 
the  liquid  from  these  vessels  to  a  large  extent ;  and  this 


PHYSIOLOGICAL   INTEGRATION    IN    ANIMALS.  369 

alfects  the  nutrition  and  efficiency  of  organs  remote  from 
the  place  of  injury.  Then  where,  as  in  further-developed 
creatures,  there  exists  an  apparatus  for  propelling  the  blood 
through  these  ramifying  channels,  injury  -of  a  single  one 
will  cause  a  loss  of  blood  that  quickly  prostrates  the  entire 
organism.  Hence  the  rise  of  a  completely-differentiated  vas- 
cular system,  is  the  rise  of  a  system  which  integrates  all 
members  of  the  body,  by  .making  each  dependent  on  the  in- 
tegrity of  the  vascular  system,  and  therefore  on  the  integrity 
of  each  member  through  which  it  ramifies.  In 

another  mode,  too,  the  establishment  of  a  distributing 
apparatus  produces  a  physiological  union  that  is  great  in 
proportion  as  this  distributing  apparatus  is  efficient.  As 
fast  as  it  assumes  a  function  unlike  the  rest,  each  part  of  an 
animal  modifies  the  blood  in  a  way  more  or  less  unlike  the 
rest,  both  by  the  materials  it  abstracts  and  by  the  products  it 
adds ;  and  hence  the  more  differentiated  the  vascular  system 
becomes,  the  more  does  it  integrate  all  parts  by  making  each 
of  them  feel  the  qualitative  modification,  of  the  blood  which 
every  other  has  produced.  This  is  simply  and  conspicuously 
exemplified  by  the  lungs.  In  the  absence  of  a  vascular 
system,  or  in  the  absence  of  one  that  is  well  marked  off 
from  the  imbedding  tissues,  the  nutritive  plasma  or  the  crude 
blood,  gets  what  small  aeration  it  can,  only  by  coming  near 
the  creature's  outer  surface,  or  those  inner  surfaces  that  are 
bathed  by  water  ;  and  it  is  probably  more  by  osmotic  ex- 
change than  in  any  other  way,  that  the  oxygenated  plasma 
slowly  permeates  the  tissues.  But  where  there  have  been 
formed  definite  channels  branching  throughout  the  body; 
and  particularly  where  there  exist  specialized  organs  for 
pumping  the  blood  through  these  channels  ;  it  manifestly 
becomes  possible  for  the  aeration  to  be  carried  on  in  one  part 
peculiarly  modified  to  further  it,  while  all  other  parts  have 
the  aerated  blood  brought  to  them.  And  how  greatly  the 
differentiation  of  the  vascular  system  thus  becomes  a  means 
of  integrating  the  various  organs,  is  shown  by  the  fatal 


ttTO  PHYSIOLOGICAL   DEVELOPMENT. 

result  that  follows   when   the   current   of  aerated   blood  is 
interrupted. 

Here,  indeed,  it  becomes  obvious  both  that  certain  physio, 
logical  differentiations  make  possible  certain  physiological 
integrations  ;  and  that,  conversely,  these  integrations  make 
possible  other  differentiations.  Besides  the  waste  products 
that  escape  through  the  lungs,  there  are  waste  products  that 
escape  through  the  skin,  the  kidneys,  the  liver.  The  blood 
has  separated  from  it  in  each  of  these  structures,  the  par- 
ticular product  which  this  structure  has  become  adapted  to 
separate ;  leaving  the  other  products  to  be  separated  by  the 
other  adapted  structures.  How  have  these  special  adaptations 
ueen  made  possible?  By  union  of  the  organs  as  recipients  of 
one  circulating  mass  of  blood.  While  there  is  no  efficient 
apparatus  for  transfer  of  materials  through  the  bodv,  the 
waste  products  of  each  part  have  to  make  their  escape  locallv; 
and  the  local  channels  of  escape  must  be  competent  to  take 
off  indifferently  all  the  waste  products.  But  it  becomes  prac- 
ticable and  advantageous  for  the  differently- localized  ex- 
creting structures,  to  become  fitted  to  separate  different  waste 
products,  as  soon  as  the  common  circulation  through  them 
grows  so  efficient  that  the  product  left  unexcreted  by  one  is 
quickly  carried  to  another  better  fitted  to  excrete  it.  So  that 
the  integration  of  them  through  a  common  vascular  system, 
is  the  condition  under  which  only  they  can  become  differen- 
tiated. How  the  specialization  of  each  is  rendered  possible 
only  by  its  connexion  with  others  that  have  become  similarly 
specialized,  we  indirectly  see  in  such  a  fact  as  that  in  chronic 
jaundice  secondary  disease  of  the  kidneys  is  apt  to  arise  in 
consequence  of  the  biliverdine  accumulated  in  the  system 
being  partly  excreted  through  them :  the  implication  being 
that  a  structure  peculiarly  fitted  to  excrete  urea  can  exist 
only  when  it  is  functionally  united  with  another  structure 
peculiarly  fitted  to  excrete  biliverdine.  Perhaps  the 

clearest  idea  of  the  way  in  which   differentiation  leads  to 
integration,   and   how,    again,    increased    integration    makes 


PHYSIOLOGICAL    INTEGRATION    IN    ANIMALS.  371 

possible  still  further  differentiation,  will  be  obtained  by  con* 
teraplating  the  analogous  dependence  in  the  social  organism. 
While  it  has  no  roads,  a  country  cannot  have  its  industries 
much  specialized :  each  locality  must  produce,  as  best  it  can, 
the  various  commodities  it  consumes,  so  long  as  it  has  no 
facilities  for  barter  with  other  localities.  But  the  localities 
being  unlike  in  their  natural  fitnesses  for  the  various  indus- 
tries, there  tends  ever  to  arise  some  exchange  of  the  commo- 
dities they  can  respectively  produce  with  least  labour.  This 
exchange  leads  to  the  formation  of  channels  of  communica- 
tion. The  currents  of  commodities  once  set  up,  make  their 
foot-paths  and  horse- tracks  more  permeable  ;  and  as  fast  as 
the  resistance  to  exchange  becomes  less,  the  currents  of 
commodities  become  greater.  Each  locality  takes  more 
of  the  products  of  adjacent  ones,  and  each  locality  devotes 
itself  more  to  the  particular  industry  for  which  it  is  naturally 
best  fitted :  the  functional  integration  makes  possible  a  further 
functional  differentiation.  This  further  functional  differen- 
tiation reacts.  The  greater  demand  for  the  special  product  of 
each  locality,  excites  improvements  in  production — leads  to 
the  use  of  methods  which  both  cheapen  and  perfect  the  com- 
modity. Hence  results  a  still  more  active  exchange ;  a  still 
clearer  opening  of  the  channels  of  communication ;  a  still 
closer  mutual  dependence.  Yet  another  influence  comes  into 
play.  As  fast  as  the  intercourse,  at  first  only  between  neigh- 
bouring localities,  makes  for  itself  better  roads — as  fast  as 
rivers  are  bridged  and  marshes  made  easily  passable,  the 
resistance  to  distribution  becomes  so  far  diminished,  that  the 
things  grown  or  made  in  each  district  can  be  profitably  carried 
to  a  greater  distance  ;  and  as  the  economical  integration  is 
thus  extended  over  a  wider  area,  the  economical  differentia- 
tion is  again  increased  ;  since  each  district,  having  a  larger 
market  for  its  commodity,  is  led  to  devote  itself  more  exclu- 
sively to  producing  this  commodity.  These  actions  and  re- 
actions continue  until  the  various  localities,  becoming  greatly 
developed  and  highly  specialized  in  their  industries,  are  at 


372  PHYSIOLOGICAL   DEVELOPMENT. 

the  same  time  functionally  integrated  by  a  network  of  roads, 
and  finally  railways,  along  which  rapidly  circulate  the  cur- 
rents severally  sent  out  and  received  by  the  localities.  And 
it  will  be  manifest  that  in  individual  organisms  a  like  corre- 
lative progress  must  have  been  caused  in  an  analogous  way. 

§  308.  Another  and  higher  form  of  physiological  integra- 
tion in  animals,  is  that  which  the  nervous  system  effects. 
Each  part  as  it  becomes  specialized,  begins  to  act  upon  the 
rest  not  only  indirectly  through  the  matters  it  takes  from 
and  adds  to  the  blood,  but  also  directly  through  the  molecular 
disturbances  it  sets  up  and  diffuses.  Whether  nerves  them- 
selves are  differentiated  by  the  molecular  disturbances  thus 
propagated  in  certain  directions,  or  whether  they  are  other- 
wise differentiated,  it  must  equally  happen  that  as  fast  as 
they  become  channels  along  which  molecular  disturbances 
travel,  the  parts  they  connect  become  physiologically  in- 
tegrated, in  so  far  that  a  change  in  one  initiates  a  change  in 
the  other.  We  may  dimly  perceive  that  if  portions  of  what 
was  originally  a  uniform  mass  having  a  common  function, 
undertake  sub-divisions  of  the  function,  the  molecular 
changes  going  on  in  them  will  be  in  some  way  complemen- 
tary to  one  another :  that  peculiar  form  of  molecular  motion 
which  the  one  has  lost  in  becoming  specialized,  the  other  has 
gained  in  becoming  specialized.  And  if  the  molecular  motion 
that  was  common  to  the  two  portions  while  they  were  undiffer- 
entiated,  becomes  divided  into  two  complementary  kinds  of 
molecular  motion ;  then  between  these  portions  there  will  be  a 
contrast  of  molecular  motions  such  that  whatever  is  plus  in 
the  one  will  be  minus  in  the  other  ;  and  hence  there  will  be  a 
special  tendency  towards  a  restoration  of  the  molecular  equili- 
brium between  the  two:  the  molecular  motion  continually 
propagated  away  from  either  will  have  its  line  of  least  resist- 
ance in  the  direction  of  the  other.  If,  as  argued 
in  the  last  chapter,  repeated  restorations  of  molecular  equili- 
brium, always  following  the  line  of  least  resistance,  tend  ever 


PHYSIOLOGICAL    INTEGRATION    IN    ANIMALS.  373 

to  make  it  a  line  of  diminished  resistance  ;  then,  in  propor- 
tion as  any  parts  become  more  physiologically  integrated  by 
the  establishment  of  this  channel  for  the  easy  transmission 
of  molecular  motion  between  them,  they  may  become  more 
physiologically  differentiated.  The  contrast  between  their 
molecular  motions  leads  to  the  line  of  discharge  ;  the  line  of 
discharge,  once  formed,  permits  a  greater  contrast  of  their 
molecular  motions  to  arise ;  thereupon  the  quantities  of 
molecular  motion  transferred  to  restore  equilibrium,  being 
increased,  the  channel  of  transfer  is  made  more  permeable  ; 
and  ics  further  permeability,  so  caused,  renders  possible  a  still 
more  marked  unlikeness  of  action  between  the  parts.  Thus 
the  differentiation  and  the  integration  progress  hand  in  hand 
as  before.  How  the  same  principle  holds  through- 

out the  higher  stages  of  nervous  development,  can  be  seen 
only  still  more  vaguely.  Nevertheless,  it  is  comprehensible 
that  as  functions  become  further  divided,  there  will  arise  the 
need  for  sub-connexions  along  which  there  may  take  place 
secondary  equilibrations  subordinate  to  the  main  ones.  It  is 
manifest,  too,  that  whereas  the  differentiation  of  functions 
proceeds,  not  necessarily  by  division  into  two,  but  often  by 
division  into  several,  and  usually  in  such  ways  as  not  to  leave 
any  two  functions  that  are  just  complementary  to  one  another, 
the  restorations  of  equilibrium,  cannot  be  so  simple  as 
above  supposed.  And  especially  when  we  bear  in  mind  that 
many  differentiated  functions,  as  those  of  the  senses,  cannot 
be  held  complementary  to  any  other  functions  in  particular  ; 
it  becomes  manifest  that  the  equilibrations  that  have  to  be 
made  in  an  organism  of  much  heterogeneity,  are  extremely 
complex,  and  do  not  take  place  between  each  organ  and  some 
other,  but  between  each  organ  and  all  the  others.  The  pecu- 
liarity of  the  molecular  motion  propagated  from  each  organ, 
has  to  be  neutralized  by  some  counter-peculiarity  in  the 
average  of  the  molecular  motions  with  which  it  is  brought 
into  relation.  All  the  variously-modified  molecular  motions 
from  the  various  parts,  must  have  their  pluses  and  minuses 


374  PHYSIOLOGICAL   DEVELOPMENT. 

mutually  cancelled :  if  not  locally,  then  at  some  centre  to 
which  each  unbalanced  motion  travels  until  it  meets  with 
some  opposite  unbalanced  motion  to  destroy  it.  Still,  involved 
as  these  actions  must  become,  it  is  possible  to  see  how  the 
general  principle  illustrated  by  the  simple  case  above  sup- 
posed, will  continue  to  hold.  For  always  the  molecular 
motion  proceeding  from  any  one  differentiated  part,  will  travel 
most  readily  towards  that  place  where  a  molecular  motion 
most  complementary  to  it  in  kind  exists — no  matter  whether 
this  complementary  molecular  motion  be  that  proceeding 
from  any  one  other  organ,  or  the  resultant  of  the  molecular 
motions  proceeding  from  many  other  organs.  So  that  the 
tendency  will  be  for  each  channel  of  communication  or  nerve, 
to  unite  itself  with  some  centre  or  ganglion,  where  it  comes 
into  relation  with  other  nerves.  And  if  there  be  any  parts 
of  its  peculiar  molecular  motion  uncancelled  by  the  mole- 
cular motions  it  meets  at  this  centre  ;  or  if,  as  will  pro- 
bably happen,  the  average  molecular  motion  which  it  there 
unites  to  produce,  differs  from  the  average  molecular  motion 
elsewhere  ;  then,  as  before,  there  will  arise  a  discharge  along 
another  channel  or  nerve  to  another  centre  or  ganglion,  where 
the  residuary  difference  may  be  cancelled  by  the  differences 
it  meets  ;  or  from  whence  it  may  be  still  further  propagated 
till  it  is  -so  cancelled.  Thus  there  will  be  a  tendency  to  a 
general  nervous  integration  keeping  pace  with  the  differen- 
tiation. 

Of  course  this  must  be  taken  as  nothing  more  than  the 
indication  of  initial  tendencies — not  as  an  hypothesis  suffi- 
cient to  account  for  all  the  facts.  It  leaves  out  of  sight  the 
origin  and  functions  of  ganglia,  considered  as  something 
more  than  nerve-junctions.  Were  there  only  these  lines  of 
easy  transmission  of  molecular  disturbance,  a  change  set  up 
in  one  organ  could  never  do  more  than  produce  its  equivalent 
of  change  in  some  other  or  others  ;  and  there  could  be  none 
of  that  large  amount  of  motion  initiated  by  a  small  sensation, 
which  we  habitually  see.  The  facts  show,  unmistakably,  that 


PHYSIOLOGICAL    INTEGRATION    IN    ANIMALS.  375 

the  slight  disturbance  communicated  to  a  ganglion,  causes  an 
overthrow  of  that  highly-unstable  nervous  matter  contained 
in  it,  and  a  discharge  from  it  of  the  greatly- increased  quantity 
of  molecular  motion  so  generated.  This,  however,  is  beyond 
our  immediate  topic.  All  we  have  here  to  note  is  the  inter- 
dependence and  unification  of  functions  that  naturally  follow 
the  differentiation  of  them. 

§  309.  Something  might  be  added  concerning  the 
further  class  of  integrations  by  which  organisms  are  con- 
stituted mechanically-coherent  wholes.  Currying  further 
certain  of  the  arguments  contained  in  the  last  chapter,  it 
might  be  not  unreasonably  inferred  that  the  binding  together 
of  parts  by  bones,  muscles,  and  ligaments,  is  a  secondary  result 
of  those  same  actions  by  which  bones,  muscles,  and  ligaments 
are  specialized.  But  adequate  treatment  of  this  division  of 
the  subject  is  at  present  scarcely  possible. 

AVhat  little  of  fact  and  inference  has  been  above  set  down, 
will,  however,  serve  to  make  comprehensible  the  general  truths 
respecting  which,  in  their  main  outlines,  there  can  be  no 
question.  Beginning  with  the  feebly-differentiated  sponge, 
of  which  the  integration  is  also  so  feeble  that  cutting  off  a 
piece  interferes  in  no  appreciable  degree  with  the  activity 
and  growth  of  the  rest,  it  is  undeniable  that  the  advance 
is  through  stages  in  which  the  multiplication  of  unlike  parts 
having  unlike  actions,  is  accompanied  by  an  increasing  inter- 
dependence of  the  parts  and  their  actions  ;  until  we  come  to 
structures  like  our  own,  in  which  a  slight  change  initiated  in 
one  part  will  instantly  and  powerfully  affect  all  other  parts — 
will  convulse  an  immense  number  of  muscles,  send  a  wave  of 
contraction  through  all  the  blood-vessels,  awaken  a  crowd  of 
ideas  with  an  accompanying  gush  of  emotions,  affect  the 
action  of  the  lungs,  of  the  stomach,  and  of  all  the  secreting 
organs.  And  while  it  is  a  manifest  necessity  that  along  with 
this  subdivision  of  functions  which  the  higher  organisms  show 
us,  there  must  be  this  close  co-ordination  of  them,  the  fore- 


376  PHYSIOLOGICAL   DEVELOPMENT . 

going  paragraphs  suggest  how  this  necessary  correlation  is 
brought  about.  For  a  great  part  of  the  physiological  union 
that  accompanies  the  physiological  specialization,  there 
appears  to  be  a  sufficient  cause  in  the  process  of  direct  equili- 
bration ;  and  indirect  equilibration  may  be  fairly  presumed  a 
sufficient  cause  for  that  which  remains. 


CHAPTER  X. 

SUH1IARY  OF   PHYSIOLOGICAL  DEVELOPMEIIT. 

§  310.  Intercourse  between  each  part  and  the  particular 
conditions  to  which  it  is  exposed,  either  habitually  in  the 
individual  or  occasionally  in  the  race,  thus  appears  to  be  the 
origin  of  physiological  development ;  as  we  found  it  to  be  the 
origin  of  morphological  development.  The  unlikenesses  of 
form  that  arise  among  members  of  an  aggregate  that  were 
originally  alike,  we  traced  to  unlikenesses  in  the  incident  forces. 
And  in  the  foregoing  chapters  we  have  traced  to  unlikenesses 
in  the  incident  forces,  those  unlikenesses  of  minute  structure 
and  chemical  composition  that  simultaneously  arise  among 
the  parts. 

In  summing  up  the  special  truths  illustrative  of  this 
general  truth,  it  will  be  proper  here  to  contemplate  more 
especially  their  dependence  on  first  principles.  Dealing  with 
biological  phenomena  as  phenomena  of  evolution,  we  have  to 
interpret  not  only  the  increasing  morphological  heterogeneity 
of  organisms,  but  also  their  increasing  physiological  hetero- 
geneity, in  terms  of  the  re-distribution  of  matter  and  motion. 
While  we  make  our  rapid  re-survey  of  the  facts,  let  us  then 
more  particularly  observe  how  they  are  subordinate  to  the 
universal  course  of  this  re-distribution. 

§  311.  The  instability  of  the  homogeneous,  or,  strictly 
Bpeaking,  the  inevitable  lapse  of  the  more  homogeneous  into 
the  less  homogeneous,  which  we  before  saw  endlessly  exeoi- 


378  PHYSIOLOGICAL    DEVELOPMENT. 

plified  by  the  morphological  differentiations  of  the  parts  of 
organisms,  we  have  here  seen  afresh  exemplified  in  ways  also 
countless,  by  the  physiological  differentiations  of  their  parts. 
And  in  the  one  case  as  in  the  other,  this  change  from  uni- 
formitv  into  multiformity  in  organic  aggregates,  is  caused,  as 
it  is  in  all  inorganic  aggregates,  by  the  necessary  exposure 
of  their  component  parts  to  actions  unlike  in  kind  or  quan- 
tity or  both.  General  proof  of  (his  is  furnished  by  the  order 
in  which  the  differences  appear.  If  parts  are  rendered 
physiologically  heterogeneous  by  the  heterogeneity  of  the 
incident  forces  ;  then  the  earliest  contrasts  should  be  between 
parts  that  are  the  most  strongly  contrasted  in  their  relations 
to  incident  forces ;  the  next  earliest  contrasts  should  occur 
where  there  are  the  next  strongest  contrasts  in  these  relations  ; 
and  so  on.  It  turns  out  that  they  do  this. 

Everywhere  the  differentiation  of  outside  from  inside 
comes  first.  In  the  simplest  plants  the  unlikeness  of 
the  cell-wall  to  the  cell-contents  is  the  conspicuous  trait  of 
structure.  The  contrasts  seen  in  the  simplest  animals  are 
of  the  same  kind  :  the  film  that  covers  a  Rhizopod  and  the 
more  indurated  coat  of  an  Infusorium,  are  more  unlike  the 
contained  sarcode  than  the  other  parts  of  this  are  from  one 
another ;  and  the  tendenc}'  during  the  life  of  the  animal  is 
for  the  unlikeness  to  become  greater.  What  is  true 

rfProiophyta  and  Protozoa,  is  true  of  the  germs  of  all  organ- 
isms up  to  the  highest :  the  differentiation  of  outer  from  inner 
is  the  first  step.  When  the  endochrome  of  an  A/(/a~cvll  has 
broken  up  into  the  clusters  of  granules  which  are  eventually 
to  become  spores,  each  of  these  quickly  acquires  a  mem- 
branous coating ;  constituting  an  unlikeness  between  surface 
and  centre.  Similarly  with  the  ovule  of  every  higher  plant : 
the  mass  of  cells  forming  it,  early  exhibits  an  outside  layer  of 
cells  distinguished  from  the  cells  within.  With  animal  germs 
it  is  the  same.  Be  it  in  a  ciliated  gemmule,  be  it  in  the 
pseud-ova  of  Aphides  and  of  the  Ceeidomyia,  or  be  it  in 
true  ova,  the  primary  differentiation  conforms  to  the  relations 


SUMMARY   OF   PHYSIOLOGICAL   DEVELOPMENT.  u79 

of  exterior  and  interior.  If  we  turn  to  adult  or- 

ganisms, vegetal  or  animal,  we  see  that  whether  they  do  or 
do  not  display  other  contrasts  of  parts,  they  always  display 
this  contrast.  Though  otherwise  almost  homogeneous,  such 
Fungi  as  the  Puff-ball,  or,  among  AlgcB,  all  which  have  a 
thullus  of  any  thickness,  present  marked  differences  between 
those  of  their  cells  which  are  in  immediate  contact  with  the 
environment  and  those  which  are  not.  Such  differences  they 
present  in  common  with  every  higher  plant ;  which, 
here  in  the  shape  of  bark  and  there  in  the  shape  of 
cuticle,  has  an  envelope  inclosing  it  even  up  to  its  petals  : 
the  only  parts  not  so  inclosed,  being  those  short-lived 
terminations  of  the  fructifying  organs,  from  which  the  dis- 
integrated tissue  is  being  cast  off  to  form  the  gerins  of  new 
individuals.  In  like  manner  among  animals,  there  is  always 
either  a  true  skin  or  an  outer  coat  analogous  to  one.  Wher- 
ever aggregates  of  the  first  order  have  united  into  ag- 
gregates of  the  second  and  third  orders — wherever  they 
have  become  the  morphological  units  of  such  higher  aggre- 
gates— the  outermost  of  them  have  grown  unlike  those  lying 
within.  Even  the  Sponge  is  not  without  a  layer  that  may 
by  analogy  be  called  dermal. 

This  lapse  of  the  relatively  homogeneous  into  the  rela- 
tively heterogeneous,  first  showing  itself,  as  on  the  hypothesis 
of  evolution  it  must  do,  by  the  rise  of  an  unlikeness  between 
outside  and  inside,  goes  on  next  to  show  itself,  as  we  infer 
that  it  must  do,  by  the  establishment  of  secondary  contrasts 
among  the  outer  parts  answering  to  secondary  contrasts 
among  the  forces  falling  on  them.  So  long  as  the  whole  sur- 
face of  a  plant  remains  similarly  related  to  the  environment, 
as  in  a  Protococcus  or  a  Vohox,  it  remains  uniform  ;  but  when 
there  come  to  be  an  attached  surface  and  a  free  surface, 
these,  being  subject  to  unlike  actions,  are  rendered  unlike. 
This  is  visible  even  in  a  unicellular  Alga  when  it  becomes 
fixed  ;  it  is  shown  in  the  distinction  between  the  under 
and  upper  parts  of  ordinary  Fungi;  and  we  see  it  iu 


380  PHYSIOLOGICAL   DEVELOPMKNT. 

the  universal  difference  between  the  imbedded  ends  and  the 
exposed  ends  of  the  higher  plants.  And  then  among  the 
less  marked  contrasts  of  surface  answering  to  the  less  marked 
contrasts  in  the  incident  forces,  come  those  between  the 
upper  and  under  sides  of  leaves ;  which,  as  we  have  seen, 
vary  in  degree  as  the  contrasts  of  forces  vary  in  degree,  and 
disappear  where  these  contrasts  disappear.  Equally 

clear  proof  is  furnished  by  animals,  that  the  original  uni- 
formity of  surface  lapses  into  multiformity,  in  proportion  as 
the  actions  of  the  environment  upon  the  surface  become 
multiform.  In  a  Worm,  burrowing  through  damp  soil  that 
acts  equally  on  all  its  sides,  or  in  a  Tfenia,  uniformly  bathed 
by  the  contents  of  the  intestine  it  inhabits,  the  parts  of  the 
integument  do  not  appreciably  differ  from  one  another ;  but 
in  creatures  not  surrounded  by  the  same  agencies,  as  those 
that  crawl  and  those  that  have  their  bodies  partially  inclosed, 
there  are  unlikenesses  of  integument  corresponding  to  unlike- 
nesses  of  the  conditions.  A  Snail's  foot  has  an  under 
surface  not  uniform  with  the  exposed  surface  of  its  body,  and 
this  again  is  not  uniform  with  the  protected  surface.  Among 
articulate  animals  there  is  usually  a  distinction  between  the 
ventral  and  the  dorsal  aspects ;  and  in  those  of  the  Articulata 
which  subject  their  anterior  and  posterior  ends  to  different 
environing  agencies,  as  do  the  Ant-lion  and  the  Hermit-crab, 
these  become  superficially  differentiated.  Ana- 

logous general  contrasts  occur  among  the  Vertelrata.  Fish, 
though  their  outsides  are  uniformly  bathed  by  water,  have 
their  backs  more  exposed  to  light  than  their  bellies ;  and  the 
two  are  commonly  distinct  in  colour.  Where  it  is  not  the 
back  and  belly  that  are  thus  dissimilarly  conditioned,  but  the 
sides,  as  in  the  Pleuroncctidce,  then  it  is  the  sides  that  be- 
come contrasted  ;  and  there  may  be  significance  in  the  fact, 
that  those  abnormal  individuals  of  this  order  which  revert  to 
the  ancestral  undistorted  type,  and  swim  vertically,  have  the 
two  sides  alike.  In  such  higher  vertebrates  as  Reptiles,  we 
BCC  repeated  this  differentiation  of  the  upper  and  under  sur- 


SUMMARY   OF   PHYSIOLOGICAL   DEVELOPMENT.  381 

faces  :  especially  in  those  of  them  which,  like  Snakes,  ex- 
pose these  surfaces  to  the  most  diverse  actions.  Even  in 
Birds  and  Mammals  which  usually,  by  raising  the  under 
surface  considerably  above  the  ground,  greatly  diminish  the 
contrast  between  its  conditions  and  the  conditions  to  which 
the  upper  surface  is  subject,  there  still  remains  some  unlike- 
ness  of  clothing  answering  to  the  remaining  unlikeness  be- 
tween the  conditions.  Thus,  without  by  any 
means  saying  that  all  such  differentiations  are  directly 
caused  by  differences  in  the  actions  of  incident  forces,  which, 
as  before  shown  (§  294),  they  cannot  be,  it  is  clear  that 
many  of  them  are  so  caused.  It  is  clear  that  parts  of  the 
surface  exposed  to  very  unlike  environing  agencies,  become 
very  unlike ;  and  this  is  all  that  needs  be  shown. 

Complex  as  are  the  transformations  of  the  inner  parts  of 
organisms  from  the  relatively  homogeneous  into  the  rela- 
tively heterogeneous,  we  still  see  among  them  a  conformity 
to  the  same  general  order.  In  both  plants  and  animals  the 
earlier  internal  differentiations  answer  to  the  stronger  con- 
trasts of  conditions.  Plants,  absorbing  all  their 
nutriment  through  their  outer  surfaces,  are  internally  modi- 
fied mainly  by  the  transfer  of  materials  and  by  mechanical 
stress.  Such  of  them  as  do  not  raise  their  fronds  above  the 
surface,  have  their  inner  tissues  subject  to  no  marked  con- 
trasts save  those  caused  by  currents  of  sap ;  and  the  lines  of 
lengthened  and  otherwise  changed  cells  that  are  formed 
where  these  currents  run,  and  are  most  conspicuous  where 
these  currents  must  obviously  be  the  strongest,  are  the  only 
decided  differentiations  of  the  interior.  But  where,  as  in 
the  higher  Cryptogams  and  in  Phtenogams,  the  leaves  are 
upheld,  and  the  supporting  stem  is  transversely  bent  by 
the  wind,  the  inner  tissues,  subject  to  different  amounts  of 
mechanical  strain,  differentiate  accordingly  :  the  deposit  of 
dense  substance  commences  in  that  region  where  the  sap- 
containing  cells  and  canals  suffer  the  greatest  intermittent 
compressions.  Animals,  or  at  least  such  of  them 


382  PHYSIOLOGICAL    DEVELOPMENT. 

as  take  food  into  their  interiors,  are  subject  to  forces  of 
another  class  tending  to  destroy  their  original  homogeneity. 
Food  is  a  foreign  substance  which  acts  on  the  interior  as  an 
environing  object  which  touches  it  acts  on  the  exterior — is 
literally  a  portion  of  the  environment,  which,  when  swal- 
lowed, becomes  a  cause  of  internal  differentiations  as  the  rest 
of  the  environment  continues  a  cause  of  external  differentia- 
tions. How  essentially  parallel  are  the  two  sets  of  actions 
and  reactions,  we  have  seen  implied  by  the  primordial  identity 
of  the  endoderm  and  ectoderm  in  simple  animals,  and  of  the 
skin  and  mucous  membrane  in  complex  animals  (§§  288,  289). 
Here  we  have  further  to  observe  that  as  food  is  the  original 
source  of  internal  differentiations,  these  may  be  expected  to 
show  themselves  first  where  the  influence  of  the  food  is 
greatest ;  and  to  appear  later  in  proportion  as  the  parts  are 
more  removed  from  the  influence  of  the  food.  They  do  this. 
In  animals  of  low  type,  the  coats  of  the  alimentary  cavity  or 
canal,  are  more  differentiated  than  the  tissue  that  lies  between 
the  alimentary  canal  and  the  wall  of  the  body.  This  tissue 
in  the  higher  Ccelenterata,  is  a  feebly-organized  parenchyma 
traversed  by  lacunae — either  simple  channels,  or  canals  lined 
with  simple  ciliated  cells ;  and  in  the  lower  Mollusca  the 
structures  bounding  the  perivisceral  cavity  and  its  ramifying 
sinuses,  are  similarly  imperfect.  Further,  it  is  observable 
that  the  differentiation  of  this  perivisceral  sac  and  its  sinuses 
into  a  vascular  system,  proceeds  centrifugally  from  the 
region  where  the  absorbed  nutriment  enters  the  mass  of  cir- 
culating liquid,  and  where  this  liquid  is  qualitatively  more 
unlike  the  tissues  than  it  is  at  the  remoter  parts  of  the  body. 
Physiological  development,  then,  is  initiated  by  that  insta- 
bility of  the  homogeneous  which  we  have  seen  to  be  every- 
where a  cause  of  evolution  (First  Principles,  §§  109—115).  That 
the  passage  from  comparative  uniformity  of  composition  and 
minute  structure  to  comparative  multiformity,  is  set  up  in 
organic  aggregates,  as  in  all  other  aggregates,  by  the  neces- 
sary unlikenesses  of  the  actions  to  which  the  parts  are  sub- 


SUMMARY    OF    PHYSIOLOGICAL    DEVELOPMENT.  383 

ject,  is  shown  by  the  universal  rise  of  the  primary  differentia- 
tion between  the  parts  that  are  universally  most  contrasted 
in  their  circumstances,  and  by  the  rise  of  secondary  differen- 
tiations obviously  related  in  their  order  to  secondary  contrasts 
of  conditions. 

§  312.  How  physiological  development  has  all  along  been 
aided  by  the  multiplication  of  effects — how  each  differen- 
tiation has  ever  tended  to  become  the  parent  of  new  differen- 
tiations, we  have  had,  incidentally,  various  illustrations.  Let 
us  here  review  the  working  of  this  cause. 

Among  plants  we  see  it  in  the  production  of  progressively- 
multiplying  heterogeneities  of  tissue  by  progressive  increase 
of  bulk.  The  integration  of  fronds  into  axes  and  of  axes  into 
groups  of  axes,  sets  up  unlikenesses  of  action  among  the  in- 
tegrated units,  followed  by  unlikenesses  of  minute  structure. 
Each  gust  transversely  strains  the  various  parts  of  the  stem 
in  various  degrees,  and  longitudinally  strains  in  various  degrees 
the  roots  ;  and  while  there  is  inequality  of  stress  at  every  place 
in  stem  and  branch,  so,  at  every  place  in  stem  and  branch,  the 
outer  layers  and  the  successively  inner  layers  are  severally 
extended  and  compressed  to  unequal  amounts,  and  have  un- 
equal modifications  wrought  in  them.  Let  the  tree  add  to  its 
periphery  another  generation  of  the  units  composing  it,  and 
immediately  the  mechanical  strains  on  the  supporting  parts 
are  all  changed  in  different  degrees,  initiating  new  differences 
internally.  Externally,  too,  new  differences  are  initiated. 
Shaded  by  the  leaf-bearing  outer  stratum  of  shoots,  the  inner 
structures  cease  to  bear  leaves,  or  to  put  out  shoots  that 
bear  leaves ;  and  instead  of  that  green  covering  which 
they  originally  had,  become  covered  with  bark  of  increasing 
thickness.  Manifestly,  then,  the  larger  integration  of  units 
that  are  originally  simple  and  uniform,  entails  physiological 
changes  of  various  orders,  varying  in  their  degrees  at  all 
parts  of  the  aggregate.  Each  branch  which,  favourably  cir- 
cumstanced, flourishes  more  than  its  neighbours,  becomes  a 
VOL.  II.  17 


384  PHYSIOLOGICAL   DEVELOPMENT. 

cause  of  physiological  differentiations,  not  only  in  its  neigh- 
bours from  which  it  abstracts  sap  and  presently  turns  from 
leaf-bearers  into  fruit-bearers,  but  also  in  the  remoter  parts. 

That  among  animals  physiological  development  is  fur- 
thered by  the  multiplication  of  effects,  we  have  lately  seen 
proved  by  the  many  changes  in  other  organs,  which  the 
growth  or  modification  of  each  excreting  and  secreting 
organ  initiates.  By  the  abstracted  as  well  as  by  the  added 
materials,  it  alters  the  quality  of  the  blood  passing  through 
all  members  of  the  body ;  or  by  the  liquid  it  pours  into  the 
alimentary  canal,  it  acts  on  the  food,  and  through  it  on  the 
blood,  and  through,  it  on  the  system  as  a  whole  :  an  addi- 
tional differentiation  in  one  part  thus  setting  up  additional 
differentiations  in  many  other  parts ;  from  each  of  which, 
again,  secondary  'differentiating  forces  reverberate  through 
the  organism.  Or,  to  take  an  influence  of  another  order,  we 
have  seen  how  the  modified  mechanical  action  of  any  member 
not  only  modifies  that  member,  but  becomes,  by  its  reactions, 
a  cause  of  secondary  modifications — how,  for  example,  the 
burrowing  habits  of  the  common  Mole,  leading  to  an  almost 
exclusive  use  of  the  fore  limbs,  have  entailed  a  dwindling 
of  the  hind  limbs,  and  a  concomitant  dwindling  of  the 
pelvis,  which,  becoming  too  small  for  the  passage  of  the 
young,  has  initiated  still  more  anomalous  modifications. 

So  that  throughout  physiological  development,  as  in 
evolution  at  large,  the  multiplication  of  effects  has  been 
a  factor  constantly  at  work,  and  working  more  actively 
as  the  development  has  advanced.  The  secondary  changes 
wrought  by  each  primary  change,  have  necessarily  become 
more  numerous  in  proportion  as  organisms  have  become 
more  complex.  And  every  increased  multiplication  of  effects, 
further  differentiating  the  organism  and,  b}r  consequence, 
further  integrating  it,  has  prepared  the  way  for  still  higher 
differentiations  and  integrations  similarly  caused. 

§  313.  The  general  truth  next  to  be  resumed,  is  that  these 


SUMMARY   OF   PHYSIOLOGICAL   DEVELOPMENT.  385 

processes  have  for  their  limit  a  state  of  equilibrium  — proxi- 
mately  a  moving  equilibrium  and  ultimately  a  complete  equili- 
brium. The  changes  we  have  contemplated  are  but  the  con- 
comitants of  a  progressing  equilibration.  In  every  aggregate 
which  we  call  living,  as  well  as  in  all  other  aggregates,  the 
instability  of  the  homogeneous  is  but  another  name  for  the 
absence  of  balance  between  the  incident  forces  and  the  forces 
which  the  aggregate  opposes  to  them  ;  and  the  passage  into 
heterogeneity  is  the  passage  towards  a  state  of  balance.  And 
to  say  that  in  every  aggregate,  organic  or  other,  there  goes 
on  a  multiplication  of  effects,  is  but  to  say  that  one  part  which 
has  a  fresh  force  impressed  on  it,  must  go  on  changing  and 
communicating  secondary  changes,  until  the  whole  of  the 
impressed  force  has  been  used  up  in  generating  equivalent 
reactive  forces. 

The  principle  that  whatever  new  action  an  organism  is 
subject  to,  must  either  overthrow  the  moving  equilibrium  of 
its  functions  and  cause  the  sudden  equilibration  called  death, 
or  else  must  progressively  alter  the  organic  rhythms,  until, 
by  the  establishment  of  a  new  reaction  balancing  the  new 
action,  a  new  moving  equilibrium  is  produced,  applies  as 
much  to  each  member  of  an  organism  as  to  the  organism  in 
its  totality.  Any  force  falling  on  any  part  not  adapl  ed  to 
bear  it,  must  either  cause  local  destruction  of  tissue,  or  must, 
without  destroying  the  tissue,  continue  to  change  it  until  it 
can  change  it  no  further ;  that  is — until  the  modified  reaction 
of  the  part  has  become  equal  to  the  modified  action.  What- 
ever the  nature  of  the  force,  this  must  happen.  If  it  is  a 
mechanical  force,  then  the  immediate  effect  is  some  distortion 
of  the  part — a  distortion  having  for  its  limit  that  attitude 
in  which  the  resistance  of  the  structures  to  further  change  of 
position,  balances  the  force  tending  to  produce  the  further 
change  ;  and  the  ultimate  effect,  supposing  the  force  to  be  con- 
tinuous or  recurrent,  is  such  a  permanent  alteration  of  form, 
or  alteration  of  structure,  or  both,  as  establishes  a  permanent 
balance.  If  the  force  is  physico-chemical,  or  chemical,  tha 


,386  PHYSIOLOGICAL   DEVELOPMENT. 

general  result  is  still  the  same  :  the  component  molecules  of 
the  tissue  must  have  their  molecular  arrangements  changed, 
and  the  change  in  their  molecular  arrangements  must  go  on 
until  their  molecular  motions  are  so  re- adjusted  as  to  equili- 
brate the  molecular  motions  of  the  new  physico-chemical  or 
chemical  agent.  In  other  words,  the  organic  matter  com- 
posing the  part,  if  it  continues  to  be  organic  matter  at  all, 
must  assume  that  molecular  composition  which  enables  it  to 
bear,  or  as  we  say  adapts  it  to,  the  incident  forces. 

Nor  is  it  less  certain  that  throughout  the  organism  as  a 
whole,  equilibration  is  alike  the  proximate  limit  of  the  changes 
wrought  by  each  action,  as  well  as  the  ultimate  limit  of  the 
changes  wrought  by  any  recurrent  actions  or  continuous 
action.  The  ordinary  movements  every  instant  going  on,  are 
movements  towards  a  new  state  of  equilibrium.  Raising  a 
limb  causes  a  simultaneous  shifting  of  the  centre  of  gravity, 
and  such  altered  tensions  and  pressures  throughout  the  body 
as  re-adjust  the  disturbed  balance.  Passage  of  liquid  into  or 
out  of  a  tissue,  implies  some  excess  of  force  in  one  direction 
there  at  work  ;  and  ceases  only  when  the  force  so  diminishes  or 
the  counter-forces  so  increase  that  the  excess  disappears.  A 
nervous  discharge  is  reflected  and  re-reflected  from  part  to 
part,  until  it  has  all  been  used  up  in  the  re-arrangements  pro- 
duced— equilibrated  by  the  reactions  called  out.  And  what 
is  thus  obviously  true  of  every  normal  change,  is  equally  true 
of  every  abnormal  change — every  disturbance  of  the  estab- 
lished rhythm  of  the  functions.  If  such  disturbance  is  a 
single  one,  the  perturbations  set  up  by  it,  reverberating 
throughout  the  system,  leave  its  moving  equilibrium  slightly 
altered.  If  the  disturbance  is  repeated  or  persistent,  its  suc- 
cessive effects  accumulate  until  they  have  produced  a  new 
moving  equilibrium  adjusted  to  the  new  force. 

Each  re-balancing  of  actions,  having  for  its  necessary  con- 
comitant a  modification  of  tissues,  it  is  an  obvious  corollary 
that  organisms  subjected  to  successive  changes  of  conditions, 
must  undergo  successive  differentiations  and  re-differentia- 


SUMMARY   OF    PHYSIOLOGICAL   EEVELOPMEM.  387 

tions.  Direct  equilibration  in  organisms,  with  all  its  accom- 
panying structural  alterations,  is  as  certain  as  is  that  uni- 
versal progress  towards  equilibrium  of  which  it  forms  part. 
And  just  as  certain  is  that  indirect  equilibration  in  organisms 
}o  which  the  remaining  large  class  of  differentiations  is  due. 
The  development  of  favourable  variations  by  the  killing  of 
individuals  in  which  they  do  not  occur  or  are  least  marked, 
is,  as  before,  a  balancing  between  certain  local  structures  and 
the  forces  they  are  exposed  to ;  and  is  no  less  inevitable  than 
the  other. 

§  314.  In  all  which  universal  laws,  we  find  ourselves  again 
brought  down  to  the  persistence  of  force,  as  the  deepest 
knowable  cause  of  those  modifications  which  constitute 
physiological  development ;  as  it  is  the  deepest  knowable 
cause  of  all  other  evolution.  Here,  as  elsewhere,  the  per- 
petual lapse  from  less  to  greater  heterogeneity,  the  perpetual 
begetting  of  secondary  modifications  by  each  primary  modi- 
fication, and  the  perpetual  approach  to  a  temporary  balance 
on  the  way  towards  a  final  balance,  are  necessary  implica- 
tions of  the  ultimate  fact  that  force  cannot  disappear,  but 
can  only  change  its  form. 

It  is  an  unquestionable  deduction  from  the  persistence  of 
force,  that  in  every  individual  organism  each  new  incident 
force  must  work  its  equivalent  of  change  ;  and  that  where  it 
is  a  constant  or  recurrent  force,  the  limit  of  the  change  it 
works  must  be  an  adaptation  of  structure  such  as  opposes  to 
the  new  outer  force  an  equal  inner  force.  The  only  thing 
open  to  question  is,  whether  such  re -adjustment  is  inherit- 
able ;  and  further  consideration  will,  I  think,  show,  that  to 
say  it  is  not  inheritable  is  indirectly  to  say  that  force  does 
not  persist.  If  all  parts  of  an  organism  have  their  func- 
tions co  ordinated  into  a  moving  equilibrium,  such  that  every 
part  perpetually  influences  all  other  parts,  and  cannot  be 
changed  without  initiating  changes  in  ail  other  parts — if  the 
limit  of  change  is  the  establishment  of  a  complete  harmony 


388  PHYSIOLOGICAL   DEVELOPMENT. 

among  the  movements,  molecular  and  other,  of  all  parts ;  then 
among  other  parts  that  are  modified,  molecularly  or  other- 
wise, must  be  those  which  cast  off  the  germs  of  new 
organisms.  The  molecules  of  their  produced  germs  must 
tend  ever  to  conform  the  motions  of  their  components,  and 
therefore  the  arrangements  of  their  components,  to  the 
molecular  forces  of  the  organism  as  a  whole ;  and  if  this 
aggregate  of  molecular  forces  is  modified  in  its  distribution 
by  a  local  change  of  structure,  the  molecules  of  the  germs 
must  be  gradually  changed  in  the  motions  and  arrangements 
of  their  components,  until  they  are  re-adjusted  to  the  aggre- 
gate of  molecular  forces.  For  to  hold  that  the  moving  equi- 
librium of  an  organism  may  be  altered  without  altering  the 
movements  going  on  in  a  particular  part  of  it,  is  to  hold  that 
these  movements  will  not  be  affected  by  the  altered  distribu- 
tion of  forces ;  and  to  hold  this  is  to  deny  the  persistence  of 
force. 


PART   VI. 

LAWS   OF  MULTIPLICATION. 


CHAPTER  I. 

THE  FACTORS.* 

§  315.  If  organisms  have  been  evolved,  their  respective 
powers  of  multiplication  must  have  been  determined  by 
natural  causes.  Grant  that  the  countless  specialities  of 
structure  and  function  in  plants  and  animals,  have  arisen 
from  the  actions  and  reactions  between  them  and  their 
environments,  continued  from  generation  to  generation ;  and 
it  follows  that  from  these  actions  and  reactions  have  also 
arisen  those  countless  degrees  of  fertility  which  we  see 
among  them.  As  in  all  other  respects  an  adaptation  of  each 
species  to  its  conditions  of  existence  is  directly  or  indirectly 
brought  about ;  so  must  there  be  directly  or  indirectly 
brought  about  an  adaptation  of  its  reproductive  activity  to 
its  conditions  of  existence. 

We  may  expect  to  find,  too,  that  permanent  and  temporary 
differences  of  fertility  have  the  same  general  interpretation. 
If  the  small  variations  of  structure  and  function  that  arise 
within  the  limits  of  each  species,  are  due  to  actions  like  those 

*  An  outline  of  the  doctrine  set  forth  in  the  following  chapters,  was 
originally  published  in  the  Westminster  Review  for  April,  1852,  under  the 
title  of,  A  Theory  of  Population  deduced  from,  the  General  Law  of  Animal 
Fertility ;  and  was  shortly  afterwards  republished  with  a  prefatory  note;  to 
the  effect  that  it  must  be  accepted  as  a  sketch  which  1  hoped  at  some  future 
time  to  elaborate.  In  now  revising  and  completing  it,  I  have  omitted  a  non- 
essential  part  of  the  argument,  while  I  have  expanded  the  remainder  by 
adding  to  the  number  of  facts  put  in  evidence,  by  meeting  objections  which 
want  of  space  before  obliged  me  to  pass  over,  and  by  drawing  various 
secondary  conclusions. 


392  LAWS   OF   MULTIPLICATION. 

which,  by  their  long-accumulating  effects,  have  produced  tho 
immense  contrasts  between  the  various  types ;  we  may  con- 
clude that,  similarly,  the  actions  to  which  changes  in  the 
rate  of  multiplication  of  each  species  are  due,  also  produce, 
in  great  periods  of  time,  the  enormous  differences  between 
the  rates  of  multiplication  of  different  species. 

Before  inquiring  in  what  ways  the  rapidities  of  increase  are 
adjusted  to  the  requirements,  both  temporary  and  permanent, 
it  will  be  needful  to  look  at  the  factors.  Let  us  set  down 
first  those  which  belong  to  the  environment,  and  then  those 
which  belong  to  the  organism. 

|  316.  Every  living  aggregate  being  one  of  which  the 
inner  actions  are  adjusted  to  balance  outer  actions,  it  follows 
that  the  maintenance  of  its  moving  equilibrium  depends  on 
its  exposure  to  the  right  amounts  of  these  actions.  Its 
moving  equilibrium  may  be  overturned  if  one  of  these  actions- 
is  either  too  great  or  too  small  in  amount ;  and  it  may  be  so 
overturned  either  by  excess  or  defect  of  some  inorganic 
agency  in  its  environment,  or  by  excess  or  defect  of  some 
organic  agency. 

Thus  a  plant,  constitutionally  fitted  to  a  certain  warmth 
and  humidity,  is  killed  by  extremes  of  temperature,  as  well 
as  by  extremes  of  drought  and  moisture.  It  may  dwindle 
away  from  want  of  soil,  or  die  from  the  presence  of  too  great 
or  too  small  a  quantity  of  some  mineral  substance  which  the 
soil  supplies  to  it.  In  like  manner,  every  animal  can  main- 
tain the  balance  of  its  functions  so  long  only  as  the  environ- 
ment adds  to  or  deducts  from  its  heat  at  rates  not  exceeding 
definite  limits.  Water,  too,  must  be  accessible  in  amount 
sufficient  to  compensate  its  loss  :  if  the  parched  air  is  rapidly 
abstracting  its  liquid  which  there  is  no  pool  or  river  to 
restore,  its  functions  cease  ;  and  if  it  is  an  aquatic  creature, 
drought  may  kill  it  either  by  drying  up  its  medium  or  by 
giving  it  a  medium  inadequately  aerated.  Thus  each  organ- 
ism, adjusted  to  a  certain  average  in  the  actions  of  its 


THE    FACTORS.  393 

inorganic  environment,  or  rather,  we  should  say,  adjusted  to 
certain  moderate  deviations  from  this  average,  is  destroyed 
by  extreme  deviations.  So,  too,  is  it  with  the 

environing  organic  agencies.  Among  plants,  only  the  para- 
sitic kinds  depend  for  their  individual  preservation  on  the 
presence  of  certain  other  organisms  (though  the  presence  of 
certain  other  organisms  is  needful  to  most  plants  for  the 
preservation  of  the  race  by  aiding  fertilization).  Here,  for 
the  continuance  of  individual  life,  particular  organisms  must 
be  absent  or  not  very  numerous — beasts  that  browse,  cater- 
pillars that  devour  leaves,  aphides  that  suck  juices.  Among 
animals,  however,  the  maintenance  of  the  functional  balance 
is  both  positively  and  negatively  dependent  on  the  amounts 
of  surrounding  organic  agents.  There  must  be  an  accessible 
sufficiency  of  the  plants  or  animals  serving  for  food  ;  and  of 
organisms  that  are  predatory  or  parasitic  or  otherwise  detri 
mental,  the  number  must  not  pass  a  certain  limit. 

This  dependence  of  the  moving  equilibrium  in  every  indi- 
vidual organism  on  an  adjustment  of  its  forces  to  the  forces 
of  the  environment,  and  the  overthrow  of  this  equilibrium 
by  failure  of  the  adjustment,  is  comprehensive  of  all  cases. 
At  first  sight  it  does  not  seem  to  include  what  we  call  natural 
death ;  but  only  death  by  violence,  or  starvation,  or  cold,  or 
drought.  But  in  reality  natural  death,  no  less  than  every 
other  kind  of  death,  is  caused  by>  the  failure  to  meet  some 
outer  action  by  a  proportionate  inner  action.  The  apparent 
difference  is  due  to  the  fact  that  in  old  age,  when  the 
quantity  of  force  evolved  in  the  organism  gradually  dimi- 
nishes, the  momentum  of  the  functions  becomes  step  by  step 
less,  and  the  variations  of  the  external  forces  relatively 
greater;  until  there  finally  comes  an  occasion  when  some 
quite  moderate  deviation  from  the  average  to  which  the 
feeble  moving  equilibrium  is  adjusted,  produces  in  it  a  fatal 
perturbation. 

§  317.  The  individuals  of  every  species  being  thus  depend- 


394  LAWS   OF   MULTIPLICATION. 

ent  on  certain  environing  actions  ;  and  severally  having  their 
moving  equilibria  sooner  or  later  overthrown  by  one  or  other 
of  these  environing  actions ;  we  have  next  to  consider  in 
what  ways  the  environing  actions  are  so  met  as  to  prevent 
extinction  of  the  species.  There  are  two  essentially  different 
ways.  There  may  be  in  each  individual  a  small  or  great 
ability  to  adjust  itself  to  variations  of  the  agencies  around 
it  and  to  a  small  or  great  number  of  such  varying  agencies 
— there  may  be  little  or  much  power  of  preserving  the 
balance  of  the  functions.  And  there  may  be  much  or  little 
power  of  producing  new  individuals  to  replace  those  whose 
moving  equilibria  have  been  overthrown.  A  few  facts  must 
oe  set  down  to  enforce  these  abstract  statements. 

There  are  both  active  and  passive  adaptations  by  which 
organisms  are  enabled  to  survive  adverse  influences.  Plants 
show  us  but  few  active  adaptations  :  that  of  the  Pitcher-plant 
and  those  of  the  reproductive  parts  of  some  flowers  (which  do 
not,  however,  conduce  to  self-preservation)  are  exceptional 
instances.  But  plants  have  various  passive  adaptations  ;  as 
thorns,  stinging  hairs,  poisonous  and  acrid  juices,  repugnant 
odours,  and  the  woolliness  or  toughness  that  makes  their  leaves 
uneatable.  Animals  exhibit  far  more  numerous 

adjustments,  both  passive  and  active.  In  some  cases  they 
survive  desiccation,  they  hybernate,  they  acquire  thicker 
clothing,  and  so  are  fitted  to  bear  unfavourable  inorganic 
actions ;  and  they  are  in  many  cases  fitted  passively  to  meet 
the  adverse  actions  of  other  organisms,  by  bearing  spines  or 
armour  or  shells,  by  simulating  neighbouring  objects  in  colour 
or  form  or  both,  by  emitting  disagreeable  odours,  or  by  having 
disgusting  tastes.  In  still  more  numerous  ways  they  actively 
contend  with  unfavourable  conditions.  Against  the  seasons 
they  guard  by  storing  up  food,  by  secreting  themselves  in 
crevices,  or  by  forming  burrows  and  nests.  They  save  them- 
selves from  enemies  by  developed  powers  of  locomotion,  taking 
the  shape  of  swiftness  or  agility  or  aptitude  for  changing 
their  media ;  by  their  strength  either  alone  or  aided  by  wea- 


THE    FACTORS.  395 

pons ;  lastly  by  their  intelligence,  without  which,  indeed, 
their  other  superiorities  would  avail  them  little.  And  then 
these  various  active  powers  serving  for  defence,  become,  in 
other  cases,  the  powers  that  enable  animals  to  aggress,  and  to 
preserve  their  lives  by  the  success  of  their  aggressions. 

The  second  process  by  which  extinction  is  prevented— the 
formation  of  new  individuals  to  replace  the  individuals 
destroyed — is  carried  on,  as  described  in  the  chapter  on 
"  Genesis,"  by  two  methods,  the  sexual  and  the  asexual. 
Plants  multiply  by  spontaneous  fission,  by  gemmation,  by 
proliferation,  and  by  the  evolution  of  young  ones  from  de- 
tached cells  and  scales  and  leaves ;  and  they  also  multiply 
by  the  casting  off  of  spores  and  sporangia  and  seeds.  In  like 
manner  among  animals,  there  are  varied  kinds  of  agamo- 
genesis,  from  spontaneous  fission  up  to  parthenogenesis,  all  of 
them  conducing  to  rapid  increase  of  numbers ;  and  we  have 
the  more  familiar  process  of  gamogenesis,  also  carried  on 
in  a  great  variety  of  ways.  This  formation  of 

new  individuals  to  replace  the  old,  is,  however,  inadequately 
conceived  if  we  contemplate  only  the  number  born  or  detached 
on  each  occasion.  There  are  four  factors,  all  variable,  on 
which  the  rate  of  multiplication  depends.  The  first  is  the 
age  at  which  reproduction  commences ;  the  second  is  the 
frequency  with  which  broods  are  produced ;  the  third  is  the 
number  contained  in  each  brood  ;  and  the  fourth  is  the  length 
of  time  during  which  the  bringing  forth  of  broods  con- 
tinues. There  must  be  taken  into  account  a  further  element 
—  the  amount  of  aid  given  by  the  parent  to  each  germ  in  the 
shape  of  stored-up  nutriment,  continuous  feeding,  warmth, 
protection,  &c. :  on  which  amount  of  aid,  varying  between 
immensely  wide  limits,  depends  the  number  of  the  new  indi- 
viduals that  survive  long  enough  to  replace  the  old,  and 
perform  the  same  reproductive  process. 

Thus,  regarding  every  living  organism  as  having  a  moving 
equilibrium  dependent  on  environing  forces,  but  ever  liable 
to  be  overthrown  by  irregularities  in  those  forces,  and  always 


1396  LAWS   OF   MULTIPLICATION. 

so  overthrown  sooner  or  later;  we  see  that  each  kind  of 
organism  can  be  maintained  only  by  generation  of  new  indi- 
viduals with  a  certain  rapidity,  and  by  helping  them  more 
or  less  fully  to  establish  their  moving  equilibria. 

§  318.  Such  are  the  factors  with  which  we  are  here  con- 
cerned. I  have  presented  them  in  abstract  shapes,  for  the 
purpose  of  showing  how  they  are  expressible  in  general  terms 
of  force — how  they  stand  related  to  the  ultimate  laws  of  re- 
distribution of  matter  and  motion. 

For  the  purposes  of  the  argument  now  to  follow,  we  may, 
however,  conveniently  deal  with  these  factors  under  a  more 
familiar  guise.  Ignoring  their  other  aspects,  we  may  class 
the  actions  which  affect  each  race  of  organisms  as  forming 
two  conflicting  sets.  On  the  one  hand,  by  what  we  call 
natural  death,  by  enemies,  by  lack  of  food,  by  atmospheric 
changes,  &c.,  the  race  is  constantly  being  destroyed.  On  the 
other  hand,  partly  by  the  endurance,  the  strength,  the  swift- 
ness, and  the  sagacity  of  its  members,  and  partly  by  their 
fertility,  it  is  constantly  being  maintained.  These  conflicting 
pets  of  actions  may  be  generalized  as— the  forces  destructive 
of  race  and  the  forces  preservative  of  race.  So  generalizing 
them,  let  us  ask  what  are  the  necessary  implications. 


CHAPTER  II. 

A  PRIORI  PRINCIPLE. 

§  319.  The  number  of  a  species  must  at  any  time  be  either 
decreasing  or  stationary  or  increasing.  If,  generation  after 
generation,  its  members  die  faster  than  others  are  born,  the 
species  must  dwindle  and  finally  disappear.  If  its  rate  of 
multiplication  is  equal  to  its  rate  of  mortality,  there  can  be 
no  numerical  change  in  it.  And  if  the  deductions  by  death 
are  fewer  than  the  additions  by  birth,  the  species  must  be- 
come more  abundant.  These  we  may  safely  set  down  as 
necessities.  The  forces  destructive  of  race  must  be  either 
greater  than  the  forces  preservative  of  race,  or  equal  to  them, 
or  less  than  them  ;  and  there  cannot  but  result  these  effects 
on  number. 

We  are  here  concerned  only  with  races  that  continue  to 
exist ;  and  may  therefore  leave  out  of  consideration  those 
cases  in  which  the  destructive  forces,  remaining  permanently 
in  excess  of  the  preservative  forces,  cause  extinction.  Prac- 
tically, too,  we  may  exclude  the  stationary  condition  of  a 
species  ;  for  the  chances  are  infinity  to  one  against  the  main- 
tenance of  a  permanent  equality  between  the  births  and  the 
deaths.  Hence,  our  inquiry  resolves  itself  into  this: — In 
races  that  continue  to  exist,  what  laws  of  numerical  variation 
result  from  these  variable  conflicting  forces,  that  are  respec- 
tively destructive  of  race  and  preservative  of  race  ? 

§  820.  Clearly  if  the  forces  destructive  of  race,  when  once 


398  LAWS   OF   MULTIPLICATION. 

in  excess,  had  nothing  to  prevent  them  from  remaining  in 
excess,  the  race  would  disappear  ;  and  clearly  if  the  forces 
preservative  of  race,  when  once  in  excess,  had  nothing  to 
prevent  them  from  remaining  in  excess,  the  race  would  go  on 
increasing  to  infinity.  In  the  absence  of  any  compensating 
actions,  the  only  possible  avoidance  of  these  opposite  extremes 
would  be  an  unstable  equilibrium  between  the  conflicting 
forces,  resulting  in  a  perfectly  constant  number  of  the  species : 
a  state  which  we  know  does  not  exist,  and  against  the 
existence  of  which  the  probabilities  are,  as  already  said, 
infinite.  It  follows,  then,  that  as  in  every  continuously- 
existing  species,  neither  of  the  two  conflicting  sets  of  forces 
remains  permanently  in  excess ;  there  must  be  some  way  of 
stopping  that  excess  of  the  one  or  the  other  which  is  ever 
occurring. 

How  is  this  done  ?  Should  any  one  allege,  in  conformity 
with  the  old  method  of  interpretation,  that  there  is  in  each 
case  a  providential  interposition  to  rectify  the  disturbed 
balance,  he  commits  himself  to  the  supposition  that  of  the 
millions  of  species  inhabiting  the  Earth,  each  one  is  yearly 
regulated  in  its  degree  of  fertility  by  a  miracle  ;  since  in  no 
two  years  do  the  forces  which  foster,  or  the  forces  which 
check,  each  species,  remain  the  same ;  and  therefore,  in  no 
two  years  is  there  required  the  same  fertility  to  balance 
the  mortality.  Few  if  any  will  say  that  God  continually 
alters  the  reproductive  activity  of  every  parasitic  fungus  and 
every  Tape- worm  or  Trichina,  so  as  to  prevent  its  extinction 
or  undue  multiplication  ;  which  they  must  say  if  they  adopt 
the  hypothesis  of  a  supernatural  adjustment.  And  in  the 
absence  of  this  hypothesis  there  remains  only  one  other. 
The  alternative  possibility  is,  that  the  balance  of  the  pre- 
servative and  destructive  forces  is  self-sustaining — is  of  the 
kind  distinguished  as  a  stable  equilibrium :  an  equilibrium 
such  that  any  excess  of  one  of  the  forces  at  work,  itself 
generates,  by  the  deviation  it  produces,  certain  counter-forces 
that  eventually  out-balance  it,  and  initiate  an  opposite  devia- 


A    PRIORI    PRINCIPLE. 

tion.  Let  us  consider  how,  in  the  case  before  us,  such  a  stable 
equilibrium  must  be  constituted. 

§  321.  When  a  season  favourable  to  it,  or  a  diminution  of 
creatures  detrimental  to  it,  causes  any  species  to  become 
more  numerous  than  usual ;  an  immediate  increase  of  certain 
destructive  influences  takes  place.  If  it  is  a  plant,  the 
supposed  greater  abundance  itself  implies  occupation  of  the 
available  places  for  growth — an  occupation  which,  leaving 
fewer  such  places  as  the  multiplication  goes  on,  itself  becomes 
a  check  on  further  multiplication — itself  causes  a  greater 
mortality  of  seeds  that  fail  to  root  themselves.  And  after- 
wards, in  addition  to  this  passive  resistance  to  continued 
increase,  there  comes  an  active  resistance  :  the  creatures  that 
thrive  at  the  expense  of  the  species — the  larvae,  the  birds,  the 
herbivores — increase  too.  If  it  be  an  animal  that  has  grown 
more  numerous,  then,  unless  by  some  exceptional  coincidence 
a  simultaneous  and  proportionate  addition  to  the  animals  or 
plants  serving  for  food  has  occurred,  there  must  result  a 
relative  scarcity  of  food.  Enemies,  too,  be  they  beasts  of 
prey  or  be  they  parasites,  must  quickly  begin  to  multiply. 
Hence,  each  kind  of  organism,  previously  existing  in  some- 
thing like  its  normal  number,  cannot  have  its  number  raised 
without  a  rise  of  the  destructive  forces,  negative  and  positive, 
quickly  commencing.  Both  negative  and  posi- 

tive destructive  forces  must  augment  until  this  increase  of 
the  species  is  arrested.  The  competition  for  places  on  which 
to  grow,  if  the  species  be  vegetal,  or  for  food  if  it  be  animal, 
must  become  more  intense  as  the  over-peopling  of  the  habitat 
progresses ;  until  there  is  reached  the  limit  at  which  the 
mortality  equals  the  reproduction.  And  as,  at  the  same 
time,  enemies  will  multiply  with  a  rapidity  which  soon 
brings  them  abreast  of  the  augmented  supply  of  prey,  the 
positive  restraint  they  exert  will  help  to  bring  about  an 
earlier  arrest  of  the  expansion  than  pressure  of  population 
alone  would  cause.  One  more  inference  may  be 


100  LAWS   OF   MULTIPLICATION. 

drawn.  Had  the  species  to  meet  no  repressing  influence 
save  that  negative  one  of  relatively-diminished  space  or 
relatively-diminished  food-supply,  the  cause  leading  to  its 
increase  might  carry  it  up  to  the  limit  set  by  this,  and  there 
leave  it:  its  enlarged  number  might  be  permanent.  But 
the  positive  repressing  influence  that  has  been  called  into 
existence,  will  prevent  this.  For  the  increase  of  enemies, 
commencing,  as  it  must,  after  the  increase  of  the  species, 
and  advancing  in  geometrical  progression  until  it  is  itself 
checked  in  like  manner,  will  end  in  an  excess  of  enemies. 
Whereupon  must  result  a  mortality  of  the  species  greater 
than  its  multiplication — a  decrease  which  will  continue  until 
its  habitat  is  underpeopled,  its  unduly-numerous  enemies 
decimated  by  starvation,  and  the  destroying  agencies  so 
reduced  to  a  minimum.  Whence  will  follow  another  in- 
crease. 

Thus,  as  before  indicated  (First  Prin.  §  §  96,  133),  there  is 
here,  as  wherever  antagonistic  forces  are  in  action,  an  alter- 
nate predominance  of  each,  causing  a  rhythmical  movement 
—  a  rhythmical  movement  which  constitutes  a  moving  equili- 
brium in  those  cases  where  the  forces  are  not  dissipated  with 
appreciable  rapidity,  or  are  re-supplied  as  fast  as  they  are 
dissipated.  While,  therefore,  on  the  one  hand,  we  see  that 
the  continued  existence  of  a  species  necessarily  implies  some 
action  by  which  the  destructive  and  preservative  forces  are 
self- adjusted  ;  we  see,  on  the  other  hand,  that  such  an  action 
is  an  inevitable  consequence  of  the  universal  process  of 
equilibration. 

§  322.  Is  this  the  sole  equilibration  that  must  exist  ? 
Clearly  not.  The  temporary  compensating  adjustments  of 
multiplication  to  mortality  in  each  species,  are  but  intro- 
ductory to  the  permanent  compensating  adjustments  of  mul- 
tiplication to  mortality  among  species  in  general.  The  above 
reasoning  would  hold  just  as  it  now  does,  were  all  species 
equally  prolific  and  all  equally  short-lived.  It  yields  no 


A    PRIORI   PRINCIPLE.  401 

answer  to  the  inquiries — why  do  their  fertilities  differ  so 
enormously,  or  why  do  their  mortalities  differ  so  enormously  P 
and  how  is  the  general  fertility  adapted  to  the  general  mor- 
tality in  each  ?  The  balancing  process  we  have  contemplated, 
can  go  on  only  within  moderate  limits — must  fail  entirely  in 
the  absence  of  a  due  proportion  between  the  ordinary  birth- 
rate and  the  ordinary  death-rate.  If  the  reproduction  of 
mice  proceeded  as  slowly  as  the  reproduction  of  men,  mice 
would  be  extinct  before  a  new  generation  could  arise :  even 
did  their  natural  lives  extend  to  fifteen  or  sixteen  years,  it 
would  still  be  extremely  improbable  that  any  would  for  so 
long  survive  all  the  dangers  they  are  exposed  to.  Con- 
versely, did  oxen  propagate  as  fast  as  infusoria,  the  race 
would  die  of  starvation  in  a  week.  Hence,  the  minor  adjust- 
ment of  varying  multiplication  to  varying  mortality  in  each 
species,  implies  some  major  adjustment  of  average  multipli- 
cation to  average  mortality.  What  must  this  adjustment  be? 

We  have  already  seen  that  the  forces  preservative  of  race 
are  two — ability  in  each  member  of  the  race  to  preserve 
itself,  and  ability  to  produce  other  members — power  to  main- 
tain individual  life,  and  power  to  generate  the  species. 
These  must  vary  inversely.  When,  from  lowness  of  organi- 
zation, the  ability  to  contend  with  external  dangers  is  small, 
there  must  be  great  fertility  to  compensate  for  the  conse- 
quent mortality ;  otherwise  the  race  must  die  out.  When, 
on  the  contrary,  high  endowments  give  much  capacity  of 
self-preservation,  a  correspondingly-low  degree  of  fertility  is 
requisite.  Given  the  dangers  to  be  met  as  a  constant  quan- 
tity; then,  as  the  ability  of  any  species  to  meet  them  must 
be  a  constant  quantity  too,  and  as  this  is  made  up  of  the  two 
factors — power  to  maintain  individual  life  and  power  to  mul- 
tiply— these  cannot  do  other  than  vary  inversely :  one  must 
decrease  as  the  other  increases. 

It  needs  but  to  conceive  the  results  of  nonconformity  to 
this  law,  to  see  that  every  species  must  either  conform  to  it 
or  cease  to  exist.  Suppose,  first,  a  species  whose  individuals 


402 


LAWS  OF   MULTIPLICATION. 


having  but  small  self-preservative  powers  are  rapidly  de- 
stroyed, to  be  at  the  same  time  without  reproductive  powers 
proportionately  great.  The  defect  of  fertility,  if  extreme, 
will  result  in  the  death  of  one  generation  before  another  has 
grown  up.  If  less  extreme,  it  will  entail  a  scarcity  such 
that  in  the  next  generation  sexual  congress  will  be  too  infre- 
quent to  maintain  even  the  small  number  that  remains  ;  and 
the  race  will  dwindle  with  increasing  rapidity.  If  still  less 
extreme,  the  consequent  degree  of  rareness,  while  not  so 
great  as  to  prevent  an  adequate  number  of  procreative 
unions,  will  be  so  great  as  to  render  special  food  very  abundant 
and  special  enemies  very  few — will  thus  diminish  the  destruc- 
tive forces  so  much  that  the  self -preservative  forces  will  be- 
come relatively  great :  so  great,  relatively,  that  when  com- 
bined with  the  small  ability  to  propagate  the  species,  they 
will  suffice  to  balance  the  small  destructive  forces.  Suppose, 
next,  a  species  whose  individuals  have  great  powers  of 
self-preservation,  while  they  have  powers  of  multiplication 
much  beyond  what  is  needful.  The  excess  of  fertility,  if 
extreme,  will  cause  sudden  extinction  of  the  species  by 
starvation.  If  less  extreme,  it  must  produce  a  permanent 
increase  in  the  number  of  the  species  ;  and  this,  followed 
by  intenser  competition  for  food  and  augmented  number 
of  enemies,  will  involve  such  an  increase  of  the  dangers 
to  individual  life,  that  the  great  self-preserving  powers  of 
the  individuals  will  not  be  more  than  sufficient  to  cope  with 
them.  That  is  to  say,  if  the  fertility  is  relatively  too  great, 
then  the  ability  to  maintain  individual  life  inevitably  becomes 
smaller,  relatively  to  the  requirements ;  and  the  inverse  pro- 
portion is  thus  established. 

So  that  when,  from  comparing  the  different  states  of  the 
same  species,  we  go  on  to  compare  the  states  of  different  species, 
we  see  that  there  is  an  analogous  adjustment — analogous 
in  the  sense  that  great  mortality  is  associated  with  great 
multiplication,  and  small  mortality  with  small  multiplication. 
And  we  see  that  the  unlikeness  of  the  cases  consists  merely 


A    PRIORI    PRINCIPLE.  403 

in  tliis,  that  what  is  a  temporary  relation  in  the  one  is  a  per- 
manent relation  in  the  other. 

§  323.  For  the  moment  it  does  not  concern  us  to  inquire 
what  is  the  origin  of  this  permanent  relation.  That  which 
we  have  now  to  note,  is  simply  that  in  some  way  or  other 
ihere  must  be  established  an  inverse  proportion  between  the 
power  to  sustain  individual  life  and  the  power  to  produce 
new  individuals.  Here  it  is  enough  for  us  to  recognize  this 
\s  a  necessary  truth.  Whether  or  not  the  permanent  rela- 
tion is  self-adjusting  in  long  periods  of  time,  as  the  tempo- 
rary relation  is  self-adjusting  in  short  periods  of  time,  is  a 
separate  question.  The  purpose  of  this  chapter  is  fulfilled  by 
showing  that  such  a  permanent  relation  must  exist. 

But  having  recognized  the  a  priori  principle  that  in  races 
which  continuously  survive,  the  forces  destructive  of  race 
must  be  equilibrated  by  the  forces  preservative  of  race ;  and 
that  supposing  these  are  constant,  there  must  be  an  inverse 
proportion  between  self-preservation  and  race- preservation  ; 
we  may  go  on  to  inquire  how  this  relation,  necessary  in 
theory,  arises  in  fact.  Leaving  out  the  untenable  hypothesis 
of  a  supernatural  pre-adj  ustment,  we  have  to  ask  in  what 
way  an  adjustment  comes  about  as  a  result  of  Evolution. 
Is  it  due  to  the  survival  of  varieties  in  which  the  proportion 
of  fertility  to  mortality  happens  to  be  the  best  ?  Or  is  the 
fertility  adapted  to  the  mortality  in  a  more  direct  way  P  To 
these  questions  let  as  now  address  ourselves. 


CHAPTER  IU. 

OBVERSE  A  PRIORI  PRINCIPLE. 

§  324.  When  dealing  with  its  phenomena  inductively,  we 
saw  that  however  it  may  be  carried  on,  Genesis  "  is  a  process 
of  negative  or  positive  disintegration ;  and  is  thus  essentially 
opposed  to  that  process  of  integration,  which  is  one  element 
of  individual  evolution."  (§  76.)  Each  new  individual,  whe- 
ther separated  as  a  germ  or  in  some  more-developed  form,  is  a 
deduction  from  the  mass  of  a  pre-existing  individual  or  of  two 
pre-existing  individuals.  Whatever  nutritive  matter  is  stored 
up  along  with  the  germ,  if  it  be  deposited  in  the  shape  of  an 
egg,  is  so  much  nutritive  matter  lost  to  the  parent.  No 
drop  of  blood  can  be  absorbed  by  the  ftfitus,  and  no  draught 
of  milk  sucked  by  the  young  when  born,  without  taking 
from  the  mother  tissue- forming  and  force-evolving  materials 
to  an  equivalent  amount.  And  all  subsequent  supplies  given 
to  progeny,  if  they  are  nurtured,  involve,  to  a  parent  or 
parents,  so  much  waste  in  exertion  that  does  not  bring  its 
return  in  assimilated  food. 

Conversely,  the  continued  aggregation  of  materials  into  one 
organism,  renders  impossible  the  formation  of  other  organ- 
isms out  of  those  materials.  As  much  assimilated  food  as  is 
united  into  a  single  whole,  is  so  much  assimilated  food  with- 
held from  a  plurality  of  wholes  that  might  else  have  been 
produced.  Given  the  absorbed  nutriment  as  a  constant 
quantity,  and  the  longer  the  building  of  it  up  into  a  con- 


OBVERSE    A    PRIORI    PRINCIPLE.  405 

crete  shape  goes  on,  the  longer  must  be  postponed  any  build- 
ing of  it  up  into  discrete  shapes.  And  similarly,  the  larger 
the  proportion  of  matter  consumed  in  the  functional  actions 
of  parents,  the  smaller  must  be  the  proportion  of  matter  that 
can  remain  to  establish  and  support  the  functional  actions  of 
offspring. 

Though  the  necessity  of  these  universal  relations  is  toler- 
ably obvious  as  thus  generally  stated,  it  will  be  useful  to  dwell 
for  a  brief  space  on  their  leading  aspects. 

§  325.  That  disintegration  which  constitutes  genesis,  may 
be  such  as  to  disperse  entirely  the  aggregate  which  integra- 
tion has  previously  produced — the  parent  may  dissolve  wholly 
into  progeny.  This  dissolution  of  each  aggregate  into  two 
or  many  aggregates,  may  occur  at  very  short  intervals,  in 
which  case  the  bulk  attained  can  be  but  extremely  small ;  or 
it  may  occur  at  longer  intervals,  in  which  case  a  larger  bulk 
may  be  attained. 

Instead  of  quickly  losing  its  own  individuality  in  the 
individualities  of  its  offspring,  each  member  of  the  race  may, 
after  growing  for  a  time,  have  portions  of  its  substance  begin 
to  develop  into  the  parental  shape  and  presently  detach 
themselves ;  and  the  parent,  maintaining  its  own  identity, 
may  continue  indefinitely  so  to  produce  young  ones.  Bui 
clearly,  the  earlier  it  commences  doing  this,  and  the  more 
rapidly  it  does  it,  the  sooner  must  the  increase*  of  its  own 
bulk  be  stopped. 

Or  again,  growth  and  development  continuing  for  a  long 
period* without  any  deduction  of  materials,  an  individual  of 
considerable  size  and  organization  may  result ;  and  then  the 
abstraction  of  substance  for  the  formation  of  new  individuals, 
or  rather  the  eggs  of  them,  may  be  so  great  that  as  soon  aa 
the  eggs  are  laid  the  parent  dies  of  exhaustion — dies,  that  is, 
from  an  excessive  loss  of  the  nutritive  matters  needed  for  its 
own  activities. 

Once  more,  the  deduction  of  materials  for  the  propagation 


406  LAWS   OF    MULTIPLICATION. 

of  the  species  may  be  postponed  long  enough  to  allow  of  great 
bulk  and  complex  structure  being  attained.  The  procreative 
subtraction  then  setting  in,  while  it  checks  and  presently 
stops  growth,  may  be  so  moderate  as  to  leave  vital  capital 
sufficient  to  carry  on  the  activities  of  the  parent ;  may  go 
on  as  long  as  parental  vigour  suffices  to  furnish,  without  fatal 
result,  the  materials  needed  to  produce  young  ones  ;  and  may 
cease  when  such  a  surplus  cannot  be  supplied,  leaving  the 
parental  life  to  continue. 

§  326.  The  opposite  side  of  this  antagonism  has  also 
several  aspects.  Progress  of  organic  evolution  may  be  shown 
in  increased  bulk,  in  increased  structure,  in  increased  amount 
or  variety  of  action,  or  in  combinations  of  these  ;  and  under 
any  of  its  forms  this  carrying  higher  of  each  individuality, 
implies  a  correlative  retardation  in  the  establishment  of  new 
individualities. 

Other  things  equal,  every  addition  to  the  bulk  of  an 
organism  is  an  augmentation  of  its  life.  Besides  being  an 
advance  in  integration,  it  implies  a  greater  total  of  acti- 
vities gone  through  in  the  assimilation  of  materials ;  and 
it  implies,  thereafter,  a  greater  total  of  the  vital  changes 
taking  place  from  moment  to  moment  in  all  parts  of  the 
enlarged  mass.  Moreover,  while  increased  size  is  thus,  in  so 
far,  the  expression  of  increased  life,  it  is  also,  where  the 
organism  is  active,  the  expression  of  increased  ability  to 
maintain  life — increased  strength.  Aggregation  of  sub- 
stance is  almost  the  only  mode  in  which  self- preserving 
power  is  shown  among  the  lowest  types ;  and'  even 
among  the  highest,  sustaining  the  body  in  its  integrity 
is  that  in  which  self-preservation  fundamentally  consists — is 
the  end  which  the  widest  intelligence  is  indirectly  made  to 
subserve.  While,  on  the  one  hand,  the  increase  of  tissue 
constituting  growth  is  conservative  both  in  essence  and  in 
result ;  on  the  other  hand,  decrease  of  tissue,  either  from 
injury,  dicease,  or  old  age,  is  in  both  essence  and  result  the 


OBVERSE    A    FIUOIU    PRINCIPLE.  407 

reverse.  And  if  so,  every  addition  to  individual  life  thus 
implied,  necessarily  delays  or  diminishes  the  casting  off  of 
matter  to  form  new  individuals. 

Other  things  equal,  too,  a  greater  degree  of  organization 
involves  a  smaller  degree  of  that  disorganization  shown  by 
the  separation  of  reproductive  gemmao  and  germs.  Detach- 
ment of  a  living  portion  or  portions  from  what  was  previously 
a  living  whole,  is  a  ceasing  of  co-ordination  ;  and  is  therefore 
essentially  at  variance  with  that  establishment  of  greater  co- 
ordination which  is  achieved  by  structural  development.  In 
the  extreme  cases  where  a  living  mass  is  continually  dividing 
and  subdividing,  it  is  manifest  that  there  cannot  arise  much 
physiological  division  of  labour;  since  progress  towards 
mutual  dependence  of  parts  is  prevented  by  the  parts 
becoming  independent.  Contrariwise,  it  is  equally  clear 
that  in  proportion  as  the  physiological  division  of  labour  is 
carried  far,  the  separative  process  must  be  localized  in  some 
comparatively  small  portion  of  the  organism,  where  it  may 
go  on  without  affecting  the  general  structure — must  become 
relatively  subordinate.  The  advance  that  is  shown  by 
greater  heterogeneity,  must  be  a  hindrance  to  multiplication 
in  another  way.  For  organization  entails  cost.  That  transfer 
and  transformation  of  materials  implied  by  differentiation, 
can  be  effected  only  by  expenditure  of  force ;  and  this  sup- 
poses consumption  of  digested  and  absorbed  food,  which  might 
otherwise  have  gone  to  make  new  organisms,  or  the  germs  of 
them.  Hence,  that  individual  evolution  which  consists  in 
progressive  differentiation,  as  well  as  that  which  consists  in 
progressive  integration,  necessarily  diminishes  that  species 
of  dissolution,  general  or  local,  which  propagation  of  the  racp 
exhibits. 

In  active  organisms  we  have  yet  a  further  opposition 
between  self-maintenance  and  maintenance  of  the  race.  All 
motion,  sensible  and  insensible,  generated  by  an  animal 
for  the  preservation  of  its  life,  is  motion  liberated  from 
decomposed  nutriment — nutriment  which,  if  not  thus  decoiu- 
VOL.  II.  is 


408  LAWS   OF   MULTIPLICATION. 

posed,  would  have  been  available  for  reproduction ;  or  rather 
— might  have  been  replaced  by  nutriment  fitted  for  repro- 
ductive purposes,  absorbed  from  other  kinds  of  food.  Hence, 
in  proportion  as  the  activities  increase — in  proportion  as,  by 
its  more  varied,  complex,  rapid,  and  vigorous  actions,  an 
animal  gains  power  to  support  itself  and  to  cope  with  sur- 
rounding dangers,  it  must  lose  power  to  propagate. 

§  327.  How  may  this  antagonism  be  best  expressed  in  a 
brief  way  ?  If  self-preservation  displayed  itself  in  the 
highest  organisms,  as.  it  does  in  the  lowest,  in  little  else  but 
continuous  growth  ;  and  if  race-preservation  consisted  always, 
as  it  does  often,  of  nothing  beyond  detachment  of  portions 
from  the  parental  mass ;  then  the  antagonism  would  be, 
throughout,  the  obviously-necessary  one  of  integration  and 
disintegration.  Maintenance  of  the  individual  and  propaga- 
tion of  the  species,  being  respectively  aggregative  and  separa- 
tive, it  would  be  as  self-evident  that  they  vary  inversely,  as 
it  is  self-evident  that  addition  and  subtraction  undo  one 
another.  But  though  the  simplest  types  show  us  the  opposi- 
tion of  self-maintenance  and  race-maintenance  almost  wholly 
under  this  form ;  and  though  higher  types,  up  to  the  most 
complex,  exhibit  it  to  a  great  extent  under  this  form  ;  yet,  as 
we  have  just  seen,  this  is  not  its  only  form.  The  total 
material  monopolized  by  the  individual  and  withheld  from 
the  race,  must  be  stated  as  the  quantity  united  to  form  its 
fabric,  plus  the  quantity  expended  in  differentiating  its 
fabric,  plus  the  quantity  expended  in  its  self- conserving 
actions.  Similarly,  the  total  material  devoted  to  the  race  at 
the  expense  of  the  individual,  includes  that  which  is  directly 
subtracted  from  the  parent  in  the  shape  of  egg  or  foetus,  plus 
that  which  is  directly  subtracted  in  the  shape  of  milk,  phis 
that  which  is  indirectly  subtracted  in  the  shape  of  matter 
consumed  in  the  exertions  of  fostering  the  young.  Hence 
this  inverse  variation  is  not  expressible  in  simple  terms  of 
aggregation  and  separation.  As  we  advance  to  moi-e  highly- 


OBVERSE   A   PRIORI    PRINCIPLE.  409 

evolved  organisms,  the  total  cost  of  an  individual  becomes 
very  much  greater  than  is  implied  by  the  amount  of  tissue 
composing  it.  So,  too,  the  total  cost  of  producing  each  new 
individual  becomes  very  much  greater  than  that  of  its  mere 
substance.  And  it  is  between  these  two  total  costs  that  the 
antagonism  exists. 

We  may,  indeed,  reduce  the  antagonism  to  a  form  compre- 
hensive of  all  cases,  if  we  consider  it  as  existing  between  the 
sums  of  the  forces,  latent  and  active,  used  for  the  two  pur- 
poses. The  molecules  which  make  up  a  plant  or  animal, 
have  been  formed  by  the  absorption  of  forces  directly  or 
indirectly  derived  from  the  sun ;  and  hence  the  quantity  of 
matter  raised  to  the  form  called  organic,  which  a  plant  or 
animal  presents,  is  equivalent  to  a  certain  amount  of  force. 
Another  amount  of  force  is  expressed  by  the  totality  of  its 
differentiations.  A  further  amount  of  force  is  that  dissipated 
in  its  actions.  And  in  these  three  amounts  added  together, 
we  have  the  whole  expense  of  the  individual  life.  So,  too, 
the  whole  expense  of  establishing  each  new  individual 
includes — first  the  forces  latent  in  the  substance  composing 
it  when  born  or  hatched ;  second  the  forces  latent  in  the 
prepared  nutriment  afterwards  supplied ;  and  third  the 
forces  expended  in  feeding  and  protecting  it.  These  two 
sets  of  forces  being  taken  from  a  common  fund,  it  is  manifest 
that  either  set  can  increase  only  by  decrease  of  the  other. 
If,  of  the  force  which  the  parent  obtains  from  the  environ- 
ment, much  is  consumed  in  its  own  life,  little  remains  to  be 
consumed  in  producing  other  lives ;  and,  conversely,  if  there 
is  a  great  consumption  in  producing  other  lives,  it  can  only 
be  where  comparatively  little  is  reserved  for  parental  life. 

Hence,  then,  Individuation  and  Genesis  are  necessarily 
antagonistic.  Grouping  under  the  word  Individuation  all 
processes  by  which  individual  life  is  completed  and  main- 
tained ;  and  enlarging  the  meaning  of  the  word  Genesia 
so  as  to  include  all  processes  aiding  the  formation  and  per- 
fecting of  new  individuals ;  we  see  that  the  two  are  funda- 


410  LAWS   OF   MULTIPLICATION. 

mentally  opposed.  Assuming  other  things  to  remain  the 
same — assuming  that  environing  conditions  as  to  climate, 
food,  enemies,  &c.,  continue  constant ;  then,  inevitably,  every 
higher  degree  of  individual  evolution  is  followed  by  a  lower 
degree  of  race-multiplication,  and  vice  versa.  Progress  in 
bulk,  complexity,  or  activity,  involves  retrogress  in  fertility ; 
and  progress  in  fertility  involves  retrogress  in  bulk,  com- 
plexity, or  activity. 

This  statement  needs  a  slight  qualification.  For  reasons 
to  be  hereafter  assigned,  the  relation  described  is  never  com- 
pletely maintained  ;  and  in  the  small  departure  from  it,  we 
shall  find  an  admirable  self-acting  tendency  to  further  the 
supremacy  of  the  most-developed  types.  Here,  however,  this 
hint  must  suffice :  explanation  would  carry  us  too  far  out  of 
our  line  of  argument.  For  the  present  it  will  not  lead  us 
astray  if  we  regard  this  inverse  variation  of  Individuation 
and  Genesis  as  exact. 

§  328.  Thus,  then,  the  condition  which  each  race  must 
fulfil  if  it  is  to  survive,  is  a  condition  which,  in  the  nature  of 
things,  it  ever  tends  to  fulfil.  In  the  last  chapter  we  saw 
that  a  species  cannot  be  maintained  unless  the  power  to 
preserve  individual  life  and  the  power  to  propagate  other 
individuals  vary  inversely.  And  here  we  have  seen  that, 
irrespective  of  an  end  to  be  subserved,  these  powers  cannot 
do  other  than  vary  inversely.  On  the  one  hand,  given  a 
certain  totality  of  destroying  forces  with  which  the  species 
has  to  contend ;  and  in  proportion  as  its  members  have 
severally  but  small  ability  to  resist  these  forces,  it  is  requisite 
that  they  should  have  great  ability  to  form  new  individuals, 
and  vice  versd.  On  the  other  hand,  given  the  quantity  of 
force,  absorbed  as  food  or  otherwise,  which  the  species  can 
use  to  counterbalance  these  destroying  forces ;  and  in  propor- 
tion as  much  of  it  is  expended  in  preserving  the  individual, 
little  of  it  can  be  reserved  for  producing  new  individuals 
and  rice  versa.  There  is  thus  complete  accordance  between 


OBVERSE   A    PRIORI    PRINCIPLE.  411 

fhc  requirements  considered  under  each  aspect.  The  two 
necessities  correspond. 

We  might  rest  on  these  deductions  and  their  several  corol- 
laries. Without  going  further  we  might  with  safety  assert 
the  general  truths  that,  other  things  equal,  advancing  evolu- 
tion must  be  accompanied  by  declining  fertility  ;  and  that,  in 
the  highest  types,  fertility  must  still  further  decrease  if 
evolution  still  further  increases.  We  might  be  sure  that  if, 
other  things  equal,  the  relations  between  an  organism  and  its 
environment  become  so  changed  as  permanently  to  diminish 
the  difficulties  of  self-preservation,  there  will  be  a  permanent 
increase  in  the  rate  of  multiplication  ;  and,  conversely,  that  a 
decrease  of  fertility  will  result  where  altered  circumstances 
make  self-preservation  more  laborious. 

But  we  need  not  content  ourselves  with  these  d  priori 
inferences.  If  they  are  true,  there  must  be  an  agreement 
between  them  and  the  observed  facts.  Let  us  see  how  far 
such  an  agreement  is  traceable. 


CHAPTER  IV. 

DIFFICULTIES  OF  INDUCTIVE  VERIFICATION". 

§  329.  Were  all  species  subject  to  the  same  kinds  and 
amounts  of  destructive  forces,  it  would  be  easy,  by  comparing 
different  species,  to  test  the  inverse  variation  of  Individuatioa 
and  Genesis.  Or  if  either  the  power  of  self-preservation  or 
the  power  of  multiplication  were  constant,  there  would  be 
little  difficulty  in  seeing  how  the  other  changed  as  the 
destroying  forces  changed.  But  comparisons  are  nearly 
always  partially  vitiated  by  some  want  of  parity.  Each 
factor,  besides  being  variable  as  a  whole,  is  compounded 
of  factors  that  are  severally  variable.  Not  simply  is  the  sum 
of  the  forces  destructive  of  race  different  in  every  case ;  and 
not  simply  are  both  sets  of  forces  preservative  of  race  unlike  in 
their  totalities  in  every  case  ;  but  each  is  made  up  of  actions 
that  bear  such  changing  proportions  to  one  another  as  to 
prevent  any  positive  estimation  of  its  amount. 

Before  dealing  with  the  facts  as  well  as  we  can,  it  will  be 
best  to  glance  at  the  chief  difficulties ;  so  that  we  may  see  the 
kind  of  verification  which  is  alone  possible. 

§  330.  Either  absolutely,  or  relatively  to  any  species, 
every  environment  differs  more  or  less  from  every  other. 

There  are  the  unlikenesses  of  media — air,  water,  earth, 
organic  matter  ;  severally  involving  special  resistances  to 
movement,  and  special  losses  of  heat.  There  are  the  con- 


DIFFICULTIES    OF    INDUCTIVE    VERIFICATION.  4H> 

trusts  of  climate  :  here  great  expenditure  for  the  maintenance 
of  temperature  is  needed,  and  there  very  little  ;  in  one 
zone  an  organism  is  supplied  with  abundant  light  all  the 
3rear  round,  and  in  another  only  for  a  few  months ;  this 
region  yields  an  almost  unfailing  supply  of  water,  while  that 
entails  the  exertion  of  travelling  many  miles  every  night  for 
a  draught. 

Permanent  differences  in  the  natures  and  distributions  of 
aliment  greatly  interfere  with  the  comparisons.  The  Swal- 
low goes  through  more  exertion  than  the  Sparrow  in  securing 
a  given  weight  of  food  ;  but  then  their  foods  are  dissimilar 
in  nutritive  qualities.  There  is  a  want  of  parallelism  between 
the  circumstances  of  those  herbivores  that  live  where  the 
plains  are  annually  covered  for  a  time  with  rich  herbage, 
but  afterwards  become  parched  up,  and  of  those  inhabiting 
more  temperate  regions.  Insects  whose  larvae  feed  on  an 
abundant  plant,  as  those  of  the  genus  Vanessa  on  the  Nettle, 
have  practically  an  environment  very  unlike  that  of  insects 
such  as  Dcilephila  Euphorbia),  whose  larvsB  feed  on  a  com- 
paratively rare  plant — the  Sea-Spurge. 

Again,  comparisons  between  creatures  otherwise  akin  in 
their  constitutions  and  circumstances,  are  hindered  by  ine- 
qualities in  their  relations  to  enemies.  Two  animals,  of 
which  one  is  predatory  and  has  no  foes  but  parasites,  while 
the  other  is  much  pursued,  cannot  properly  be  contrasted 
with  a  view  to  determining  the  influence  of  size  or  com- 
plexity. 

Without  multiplying  instances,  it  will  be  clear  enough 
then  that  the  aggregate  of  destructive  actions,  positive  and 
negative,  which  each  species  has  to  contend  with,  is  so 
undefinable  in  the  amounts  and  kinds  of  its  components, 
that  nothing  beyond  a  vague  idea  of  its  relative  total  can 
be  formed. 

§  331.  Besides  these  immense  variations  in  the  outer 
actions  to  be  counter-balanced,  there  are  immense  variations 


114  LAWS   OF    MULTIPLICATION. 

in  the  inner  actions  required  to  counter-balance  them.  Even 
if  species  were  similarly  conditioned,  self-preservation  would 
require  of  them  extremely  unlike  expenditures  of  force. 

Tho  cost  of  locomotion  increases  in  a  greater  ratio  than  the 
size.  In  virtue  of  the  law  that  the  weights  of  animals  increase 
as  the  cubes  of  their  dimensions,  while  their  strengths  increase 
only  as  the  squares  of  their  dimensions  (§  46),  a  given  speed 
requires  a  large  animal  to  consume  more  substance  in  propor- 
tion to  its  weight,  than  it  requires  a  small  animal  to  consume  ; 
and  this  law  holding  of  all  the  mechanical  actions,  there 
results,  other  things  equal,  a  difficulty  of  self- maintenance 
that  augments  in  a  more  rapid  ratio  than  the  bulk.  Nor 
must  we  overlook  the  further  complication,  that  among 
aquatic  creatures  the  variation  of  resistance  of  the  medium 
partially  neutralizes  this  effect. 

Again,  the  heat-consumption  is  a  changing  element  in  the 
total  expense  of  self-preservation.  Creatures  that  have  tem- 
peratures scarcely  above  that  of  the  air  or  water,  may,  other 
things  equal,  accumulate  more  surplus  nutriment  than 
creatures  that  have  to  keep  their  bodies  warm  spite  of  the 
continual  loss  by  radiation  and  conduction.  This  difference 
of  cost  is  modified  by  the  presence  or  absence  of  natural 
clothing ;  and  it  is  also  modified  by  unlikenesscs  of  size.  Here 
the  bulky  animals  have  the  advantage  :  small  masses  cool- 
ing more  rapidly  than  large  ones. 

Dissimilarities  of  attack  and  defence  are  also  causes  of 
variation  in  the  outlay  for  self-maintenance.  A  creature 
that  has  to  hunt,  as  compared  with  another  that  gets  a 
sufficiency  of  prey  by  lying  in  wait,  or  a  creature  that 
escapes  by  speed  as  compared  with  another  that  escapes  by 
concealment,  obviously  leads  a  life  that  is  physiologically 
more  costly.  Animals  that  protect  themselves  passively, 
as  the  TIedge-hog  by  its  spines  or  as  the  Skunk  and  the 
Musk-rat  by  their  intolerable  odours,  are  relatively  econo- 
mical ;  and  have  the  more  vital  capital  for  other  purposes. 

Amplification  is  needless.     These  instances  will  show  that 


DIFFICULTIES    OF    INDUCTIVE    VERIFICATION.  415 

anything  beyond  very  general  conceptions  of  the  individual 
expenditures  in  different  cases,  cannot  be  reached. 

§  332.  Still  more  entangled  are  we  among  qualifying  con- 
siderations when  we  contrast  species  in  their  powers  of  multi- 
plication. The  total  cost  of  Genesis  admits  of  even  less 
definite  estimation  than  does  the  total  cost  of  Individua- 
tion.  I  do  not  refer  merely  to  the  truth  that  the  degree  of 
fertility  depends  on  four  factors — the  age  of  commencing 
reproduction,  the  number  in  each  brood,  the  frequency  of  the 
broods,  and  the  time  during  which  broods  continue  to  be 
repeated.  There  are  many  further  obstacles  in  the  way  of 
comparisons. 

Were  all  multiplication  carried  on  sexually,  the  problem 
would  be  less  involved  ;  but  there  are  many  kinds  of  asexual 
multiplication  alternating  with  the  sexual.  This  asexual 
multiplication  is  in  some  cases  perpetual  instead  of  occa- 
sional ;  and  often  has  more  forms  than  one  in  the  same 
species.  The  result  is  that  we  have  to  compare  what  is  here 
a  periodic  process  with  what  is  elsewhere  a  cyclical  process 
partly  continuous  and  partly  periodic — the  calculation  of  fer- 
tility in  this  last  case  being  next  to  impossible. 

We  have  to  avoid  being  misled  by  the  assumption  that  the 
cost  of  Genesis  is  measured  by  the  number  of  young  produced, 
instead  of  being  measured,  as  it  is,  by  the  weight  of  nutri- 
ment abstracted  to  form  the  young,  plus  the  weight  con- 
sumed in  caring  for  them.  This  total  weight  may  be 
very  diversely  apportioned.  In  contrast  to  the  Cod  with  its 
million  of  small  ova  spawned  without  protection,  we  may- 
put  the  Hippocampus  or  the  Pipe-fish,  with  its  few  relatively- 
large  ova  carried  about  by  the  male  in  a  caudal  pouch,  or 
seated  in  hemispherical  pits  in  its  skin  ;  or  we  may  put  the 
still  more  remarkable  genus  Arius,  and  especially  Arius 
Boakeii — a  fish  some  six  or  seven  inches  long,  which  produces 
ten  or  a  dozen  eggs  as  large  as  marbles,  that  are  carried  by 
the  male  in  his  mouth  till  they  are  hatched.  Here  though 


416  LAWS    OF    MULT1PLICATIOX. 

the  degrees  of  fertility,  if  measured  by  the  numbers  of 
fertilized  germs  deposited,  are  extremely  unlike,  they  are 
less  unlike  if  measured  by  the  numbers  of  young  that  arc 
hatched  and  survive  long  enough  to  take  care  of  themselves  ; 
nor  will  the  tax  on  the  parent-Cod  seem  so  immensely  dif- 
ferent from  that  on  the  parent- Anus,  if  the  masses  of  the  ova, 
instead  of  their  numbers,  are  compared.  Again, 

while  sometimes  the  parental  loss  is  little  else  but  the  matter 
deducted  to  form  eargs,  &c. ;  at  other  times  it  takes  the 
shape  of  a  small  direct  deduction  joined  with  a  large  indirect 
outlay.  The  Mason- wasp  furnishes  a  t}'pical  instance.  In 
journeyings  hither  and  thither  to  fetch  bit  by  bit  the 
materials  for  building  a  cell ;  in  putting  together  these 
materials,  as  well  as  in  secreting  glutinous  matter  to  act  as 
cement ;  and  then,  afterwards,  in  the  labour  of  seeking  for, 
and  carrying,  the  small  caterpillars  with  which  it  fills  up  the 
cell  to  serve  its  larva  with  food  when  it  emerges  from  the 
ogg ;  the  Mason-wasp  probably  expends  more  substance  than 
is  contained  in  the  egg  itself.  And  this  supplementary  ex- 
penditure is  manifestly  so  great,  that  but  few  eggs  can  be 
housed  and  provisioned. 

Estimates  of  the  cost  of  Genesis  are  further  complicated  by 
variations  in  the  ratio  borne  by  the  two  sexes.  Among 
Fishes  the  mass  of  milt  approaches  in  size  the  mass  of  spawn ; 
but  amorg  higher  Vertebra  fa  the  substance  lost  by  the  one 
sex  in  the  shape  of  sperm- cells  is  small  compared  with  that 
lost  by  the  other  sex  in  the  shape  of  albumen  stored-up  in 
the  eggs,  or  blood  supplied  to  the  foetus,  or  milk  given  to  the 
young.  Then  there  come  the  differences  of  indirect  tax 
on  males  and  females.  While,  frequently,  the  fostering  of 
the  young  devolves  entirely  on  the  female,  occasionally,  the 
male  undertakes  it  wholly  or  in  part.  After  building  a 
nest,  the  male  Stickleback  guards  the  eggs  till  they  are 
hatched ;  as  does  also  the  great  Silurus  glanis  for  some  forty 
days,  during  which  he  takes  no  food.  And  then,  among  most 
birds,  we  have  the  male  occupied  in  feeding  the  female  during 


DIFFICULTIES   OF    INDUCTIVE    VERIFICATION.  417 

incubation,  and  the  young  afterwards.  Evidently  all  these 
differences  affect  the  proportion  between  the  total  cost  of  re- 
production and  the  total  cost  of  individuation. 

Whether  the  species  is  monogamous  or  polygamous,  and 
whether  there  are  marked  differences  of  size  or  of  structure 
between  males  and  females,  are  also  questions  not  to  be  over- 
looked. If  there  are  many  females  to  cne  male,  the  total 
quantity  of  assimilated  matter  devoted  by  each  generation  to 
the  production  of  a  new  generation,  is  greater  than  if  there 
is  a  male  to  each  female.  Similarly,  where  the  requirements 
are  such  that  small  males  will  suffice,  the  larger  quantity  of 
food  left  for  the  females,  makes  possible  a  greater  surplus 
available  for  reproduction.  And  where,  as  in  some  of  the 
Cirrhipcdla,  or  such  a  parasite  as  Sphcerularia  Bombi,  the 
female  is  a  thousand  or  many  thousand  times  the  size  of  the 
male,  the  reproductive  capacity  is  almost  doubled  :  the  effect 
on  the  rate  of  multiplication  being  something  like  that  which 
would  result  if  any  ordinary  race  could  have  all  its  males 
replaced  by  fertile  females.  Conversely,  where  the 

habits  of  the  race  render  it  needless  that  both  sexes  should 
have  developed  powers  of  locomotion — where,  as  in  the  Glow- 
worm and  sundry  Lepidoptera,  the  female  is  wingless  while 
the  male  has  wings — the  cost  of  Individuation  not  being  so 
great  for  the  species  as  a  whole,  there  arises  a  greater  reserve 
for  Genesis :  the  matter  which  would  otherwise  have  gone  to 
the  production  of  wings  and  the  using  of  them,  may  go  to 
the  production  of  ova. 

Other  complications,  as  those  which  we  see  in  Bees  and 
Ants,  might  be  dwelt  on ;  but  the  foregoing  will  amply  serve 
the  intended  purpose. 

§  333.  To  ascertain  by  comparison  of  cases  whether  Indi- 
viduation and  Genesis  vary  inversely,  is  thus  an  under- 
taking so  beset  with  difficulties,  that  we  might  despair  of  any 
satisfactory  results,  were  not  the  relation  too  marked  a  one 
to  be  hidden  even  by  all  these  complexities.  Species  are 


418  LAWS   OF    MULTIPIJCATI'JN. 

BO  extremely  contrasted  in  their  degrees  of  evolution,  and  so 
extremely  contrasted  in  their  rates  of  multiplication,  that  the 
law  of  relation  between  these  characters  becomes  unmis- 
takable when  the  evidence  is  looked  at  in  its  ensemble.  This 
we  shall  soon  find  on  ranging  in  order  a  number  of  typical 
cases. 

In  doing  this  it  will  be  convenient  to  neglect,  temporarily, 
all  unlikenesses  among  the  circumstances  in  which  organ- 
isms are  placed.  At  the  outset,  we  will  turn  our  attention 
wholly  to  the  antagonism  displayed  between  the  integrative 
process  which  results  in  individual  evolution  and  the  disinte- 
grative  process  which  results  in  multiplication  of  individuals  ; 
and  this  we  will  consider  first  as  we  see  it  under  the  several 
forms  of  agamogenesis,  and  then  as  we  see  it  under  the  seve- 
ral forms  of  gamogenesis.  "We  will  next  look  at  the  anta- 
gonism between  propagation  and  that  evolution  which  is 
shown  by  increased  complexity.  And  then  we  will  consider 
the  remaining  phase  of  the  antagonism,  as  it  exists  between 
the  degree  of  fertility  and  the  degree  of  evolution  expressed 
by  activity. 

Afterwards,  passing  to  the  varying  relations  between 
organisms  and  their  environments,  we  will  note  how  relative 
increase  in  the  supply  of  "food,  or  relative  decrease  in  the 
quantity  of  force  expended  by  the  individual,  entails  relative 
increase  in  the  quantity  of  force  devoted  to  multiplication, 
and  vice  versa. 

Certain  minor  qualifications,  together  with  sundry  impor- 
tant corollaries,  may  then  be  entered  upon. 


CHAPTER  V. 

ANTAGONISM  BETWEEN  GROWTH  AND  ASEXUAL  GENESIS. 

§  334.  When  illustrating,  in  Part  IV.,  the  morphological 
composition  of  plants  and  animals,  there  were  set  down  in 
groups,  numerous  facts  which  we  have  here  to  look  at  from 
another  point  of  view.  Then  we  saw  how,  by  union  of  small 
simple  aggregates,  there  are  produced  large  compound  aggre- 
gates. Now  we  have  to  observe  the  reactive  effect  of  this 
process  on  the  relative  numbers  of  the  aggregates.  Our 
present  subject  is  the  antagonism  of  Individuation  and 
Genesis  as  seen  under  its  simplest  form,  in  the  self-evident 
truth  that  the  same  quantity  of  matter  may  be  divided  into 
many  small  wholes  or  few  large  wholes;  but  that  number 
negatives  largeness  and  largeness  negatives  number. 

In  setting  down  some  examples,  we  may  conveniently 
adopt  the  same  arrangement  as  before.  We  will  look  at  the 
facts  as  they  are  presented  by  vegetal  aggregates  of  the  first 
order,  of  the  second  order,  and  of  the  third  order  ;  and  then 
as  they  are  presented  by  animal  aggregates  of  the  same  three 
orders. 

§  335.  The  ordinary  unicellular  plants  are  at  once  micro- 
scopic and  enormously  prolific.  The  often  cited  Protococcus 
nivalis,  which  shows  its  immense  powers  of  multiplication  by 
reddening  wide  tracts  of  snow  in  a  single  night,  does  this  by 
developing  in  its  cavity  a  brood  of  young  cells,  which,  being 


120  LAWS    OF    MULTIPLICATION. 

presently  set  free  by  the  bursting  of  the  parent-cell,  severally 
grow  and  quickly  repeat  the  process.  The  like  occurs  among 
sundry  of  those  kindred  forms  of  minute  Ahjce  which,  by 
their  enormous  numbers,  sometimes  suddenly  change  pools  to 
an  opaque  green.  So,  too,  the  Dcsmidiaccce  often  multiply  so 
greatly  as  to  colour  the  water ;  and  among  the  Diatomacece 
the  rate  of  genesis  by  self- division,  "  is  something  really  extra- 
ordinary. So  soon  as  a  frustule  is  divided  into  two,  each  of 
the  latter  at  once  proceeds  with  the  act  of  self-division  ;  so 
that,  to  use  Professor  Smith's  approximative  calculation  of 
the  possible  rapidity  of  multiplication,  supposing  the  process 
to  occupy,  in  any  single  instance,  twenty-four  hours,  '  we 
should  have,  as  the  progeny  of  a  single  frustule,  the  amazing 
number  of  one  thousand  millions  in  a  single  month.' "  In 
these  cases  the  multiplication  is  so  carried  on  that  the  parent 
is  lost  in  the  offspring — the  old  individuality  disappears 
cither  in  the  swarms  of  zoospores  it  dissolves  into,  or  in  the 
two  or  four  new  individualities  simultaneously  produced  by 
fission.  Vegetal  aggregates  of  the  first  order, 

have,  however,  a  form  of  agamogenesis  in  which  the  parent 
individuality  is  not  lost :  the  young  cells  arise  from  the  old 
cells  by  external  gemmation.  This  process,  too,  repeated  as 
it  is  at  short  intervals,  results  in  immense  fertility.  The 
Yeast-fungus,  which  in  a  few  hours  thus  propagates  itself 
throughout  a  large  mass  of  wort,  offers  a  familiar  example. 

In  certain  compound  forms  that  must  be  classed  as  plants 
of  the  second  order  of  aggregation,  though  very  minute  ones, 
self-division  similarly  increases  the  numbers  at  high  rates. 
The  Sarcina  ventriculi,  a  parasitic  plant  that  infests  the 
stomach  and  swarms  afresh  as  fast  as  previous  swarms  are 
vomited,  shows  us  a  spontaneous  fission  of  clusters  of  cells. 
An  allied  mode  of  increase  occurs  in  Gonium  pectorale :  each 
cell  of  the  cluster  resolving  itself  into  a  secondary  cluster, 
and  the  secondary  clusters  then  separating.  "  Supposing, 
which  is  very  probable,  that  a  young  Gonium  after  twenty- 
four  hours  is  capable  of  development  by  fission,  it  follow8 


GROWTH    AND   ASEXUAL   GENESIS.  421 

that  under  favourable  conditions  a  single  colony  may  on  the 
second  day  develop  16,  on  the  ihird  256,  on  the  fourth  4,096, 
and  at  the  end  of  a  week  268,435,456  other  organisms  like 
itself."  In  the  Volvocince  this  continual  dissolution  of  a  primary 
compound  individual  into  secondary  compound  individuals,  is 
carried  on  endogenously — the  parent  bursting  to  liberate  tho 
young ;  and  the  numbers  arising  by  this  method,  also  are  some- 
times so  great  as  to  tint  large  bodies  of  water.  More 
fully  established  and  organized  aggregates  of  the  second 
order,  such  as  the  higher  Thallogens  and  the  lower  Acrogens, 
do  not  sacrifice  their  individualities  by  fission ;  but  never- 
theless, by  the  kindred  process  of  gemmation,  are  continually 
hindered  in  the  increase  of  their  individualities.  The  gemmae 
called  tetraspores  are  cast  off  in  great  numbers  by  the  marine 
A.1gce.  Among  those  simple  Jitngcrmanniacece  which  consist 
of  single  fronds,  the  young  ones  that  bud  out  grow  for  a  time 
in  connexion  with  their  parents,  send  rootlets  from  their 
under  sides  into  the  soil,  and  presently  separate  themselves — 
a  habit  which  augments  the  number  of  individuals  in  propor- 
tion as  it  checks  their  growths. 

Plants  of  the  third  order  of  composition,  arising  by  arrest 
of  this  separation,  exhibit  a  further  corresponding  decrease 
in  the  abundance  of  the  aggregates  formed.  Acrogens  of 
inferior  types,  in  which  the  axes  produced  by  integration  of 
fronds  are  but  small  and  feeble,  are  characterized  by  the 
habit  of  throwing  off  bulbils — bud-shaped  axes  which,  falling 
and  taking  roct,  add  to  the  number  of  distinct  individuals. 
This  agamic  multiplication,  very  general  among  the  Mosses 
and  their  kindred,  and  not  uncommon  under  a  modified 
form  in  such  higher  types  as  the  Ferns,  many  of  which 
produce  young  ones  from  the  surfaces  of  their  fronds,  becomes 
very  unusual  among  Phacnogams.  The  detachment  of  bulbils, 
though  not  unknown  among  them,  is  exceptional.  And  while 
it  is  true  that  some  flowering  plants,  as  the  Strawberry, 
multiply  by  a  process  allied  to  gemmation,  yet  this  is 
anything  but  characteristic  of  the  class.  A  leading  trait  oi 


422  LAWS   OF    MULTIPLICATION. 

these  highest  groups,  to  which  the  largest  members  of  the 
vegetal  kingdom  belong,  is  that  agamogcnesis  has  so  far 
ceased  that  it  does  not  originate  independent  plants.  Though 
the  axes  which,  budding  one  out  of  another,  compose  a  tree, 
are  the  equivalents  of  asexually-produced  individuals ;  yet 
the  asexual  production  of  them  stops  short  of  separation. 
These  vast  integrations  arise  where  spontaneous  disintegra- 
tion, and  the  multiplication  effected  by  it,  have  come  to  an 
end. 

Thus,  not  forgetting  that  certain  Phaenogams,  as  Begonia 
pJryllomamaca,  revert  to  quite  primitive  modes  of  increase,  we 
may  hold  it  as  beyond  question  that  while  among  the  most 
minute  plants  asexual  multiplication  is  universal,  and  pro- 
duces enormous  numbers  in  short  periods,  it  becomes  step  by 
step  more  restricted  in  range  and  frequency  as  we  advance  to 
large  and  compound  plants ;  and  disappears  so  generally 
from  the  largest,  that  its  occurrence  is  regarded  as  anomalous. 

§  336.  Parallel  examples  showing  the  inverse  variation  of 
growth  and  asexual  genesis  among  animals,  make  clear  the 
purely  quantitative  nature  of  this  relation  under  its  original 
form.  Of  the  Amoeba  it  is  said  that  "  when  a  large  variable 
process  has  been  shot  out  far  from  the  chief  mass  and  become 
enlarged  at  the  extremity,  the  expanded  end  retains  its  posi- 
tion, whilst  the  portion  connecting  it  with  the  body  becomes 
finer  and  finer  by  being  withdrawn  into  the  parent  mass, 
until  it  at  last  breaks  across,  leaving  a  detached  piece,  which 
immediately  on  its  own  account  shoots  out  processes,  and 
manifests  an  independent  existence.  This  phenomenon  is 
therefore  one  of  simple  detachment,  and  cannot  rightly  be 
called  a  process  of  fission."  .But  it  shows  us,  nevertheless, 
how  the  primordial  form  of  multiplication  is  nothing  more 
than  a  separation,  instead  of  a  continued  union,  of  the  grow- 
ing mass.  Among  the  Protozoa,  as  among  the 
Frotophyta,  there  occurs  that  process  by  which  the  in- 
dividuality of  the  parent  is  wholly  lost  in  producing  offspring 


GROWTH   AND   ASEXUAL   GENESIS.  423 

—the  breaking  up  of  the  parental  mass  into  a  number  of 
germs.  An  example  is  supplied  by  one  of  the  lowest  of  the 
class — the  Gregarina.  This  creature,  which  is  nothing  more 
than  a  minute  spheroidal  nucleated  mass  of  protoplasm, 
having  a  structureless  outer  layer  denser  than  the  rest,  but 
being  without  mouth  or  any  organ,  resolves  itself  into  a 
multitude  of  still  more  minute  masses,  which  when  set  free 
by  bursting  of  the  envelope,  shortly  become  Amoeba-form, 
and  severally  assuming  the  structure  of  the  parent,  go 
through  the  same  course.  Some  of  the  Infusoria,  as  for  in- 
stance those  of  the  genus  Kolpoda,  similarly  become  encysted 
and  subsequently  break  up  into  young  ones.  The 

more  familiar  mode  of  increase  among  these  animal-aggre- 
gates of  the  first  ordep,  by  fission,  though  it  sacrifices  the 
parent  individuality  by  merging  it  in  the  individualities  of 
the  two  produced,  sacrifices  it  less  completely  than  does  the 
dissolution  into  a  great  number  of  germs.  Occurring,  how- 
ever, as  this  fission  does,  very  frequently,  and  being  com- 
pleted, in  some  cases  that  have  been  observed,  in  the  course 
of  half-an-hour,  it  results  in  immensely- rapid  multiplication. 
If  all  its  offspring  survive,  and  continue  dividing  them- 
selves, a  single  Paramecium  is  said  to  be  capable  of  thus 
originating  268  millions  in  the  course  of  a  month.  Nor  is 
this  the  greatest  known  rate  of  increase.  Another  animalcule, 
visible  only  under  a  high  magnifying  power,  "  is  calculated 
to  generate  170  billions  in  four  days."  And  these  enormous 
powers  of  propagation  are  accompanied  by  a  minuteness  so 
extreme,  that  of  some  species  one  drop  of  water  would  contain 
as  many  individuals  as  there  are  human  beings  on  the  Earth  ! 
Making  allowance  for  exaggeration  in  these  estimates,  it  is 
beyond  question  that  among  these  smallest  of  animals  tho 
rate  of  asexual  multiplication  is  by  far  the  greatest ;  and 
this  suffices  for  the  purposes  of  the  argument.* 

*  That  these  estimated  rates  are  not  greater  than  is  probable,  may  be 
inferred  from  such  observations  as  that  of  Mr.  Brightwell  on  the  buda 
of  Zoothwrnnium,  "At  nine  in  the  morning,  one  of  these  buds,  or  ova,  was 


424  LAWS    OF    MULTIPLICATION. 

Of  animal  aggregates  belonging  to  the  second  order,  that 
multiply  asexually  with  rapidity,  the  familiar  Polypes 
furnish  conspicuous  examples.  By  gemmation  in  most 
cases,  in  other  cases  by  fission,  and  in  some  cases  by  both, 
the  agamogenesis  is  carried  on  among  these  tribes.  As 
shown  in  Fig.  148,  the  budding  of  young  ones  from  the 
parent  Hydra  is  carried  on  so  actively,  that  before  the  oldest 
of  them  is  cast  off  half-a-dozen  or  more  others  have  reached 
various  stages  of  growth ;  and  even  while  still  attached,  the 
first-formed  of  the  group  have  commenced  budding  out 
from  their  sides  a  second  generation  of  young  ones.  In  the 
Hydra  tuba  this  gemmiparous  multiplication  is  from  time  to 
time  interrupted  by  a  transverse  splitting-up  of  the  body  into 
segments,  which  successively  separate  and  swim  away :  the 
result  of  the  two  processes  being,  that  in  the  course  of  a 
season  there  are  produced  from  a  single  germ,  great  numbers 
of  young  Msdusaty  which  are  the  adult  or  sexual  forms  of  tho 
species.  Respecting  Ccelenterate  animals  of  this  degree  of 
composition,  it  may  be  added  that  when  we  ascend  to  the 
larger  kinds  we  find  asexual  genesis  far  less  active. 
Though  comparisons  are  interfered  with  by  differences  of 
structure  and  mode  of  life,  yet  the  contrasts  are  too  striking 
to  have  their  meanings  much  obscured.  If,  for  instance,  we 
take  a  solitary  Actinozoon  and  a  solitary  Hydrozoon,  we  see 
that  the  relatively-great  bulk  of  the  first,  goes  along  with  a 
relatively- slow  agamogenesis.  The  common  Sea-anemones 
are  but  occasionally  observed  to  undergo  self-division :  their 
numbers  are  not  rapidly  increased  by  this  process.  A 
higher  class  of  secondary  aggregates  exemplifies  the  same 


observed  fixed  to  the  glass  by  a  sheathed  pedicle ;  a.  ciliary  motion  becamo 
perceptible  at  the  top  of  the  bulb  ;  and  at  ten  it  had  divided  longitudinally 
into  two  buds,  each  supported  by  a  short  stalk.  The  ciliary  motion  continued 
in  the  centre  of  each  of  these  two  buds,  which  by  degrees  expanded  longitudi- 
nally, and  at  twelve  had  become  four  buds.  By  four  in  the  afternoon,  these 
four  buds  had  divided  in  like  manner  and  increased  to  nine,  with  an  elongated 
footstalk,  and  interior  contractile  muscle." 


GROWTH   AND   ASEXUAL   GENESIS.  425 

general  truth  with  a  difference.  In  the  smaller  members  the 
agamogenesis  is  incomplete,  and  in  the  larger  it  disappears. 
Each  sub-section  of  iheMolhiscoida  shows  us  this.  The  gemma- 
tion of  the  minute  Polyzoa,  though  it  does  not  end  in  the  sepa- 
ration of  the  young  individuals,  habitually  goes  to  the  extent 
of  producing  families  of  partially-independent  individuals ; 
but  their  near  allies  the  Brachiopoda,  which  immensely  exceed 
them  in  size,  are  solitary  and  not  gemmiparous.  So,  too,  is 
it  with  the  Ascidioida.  And  then  among  the  true  Mollusca, 
including  all  the  largest  forms  belonging  to  this  sub-kingdom, 
no  such  thing  is  known  as  fission  or  gemmation. 

Take  next  the  Annulosa,  including  under  this  title  the 
Annuloida.  When  treating  of  morphological  composition, 
reasons  were  given  for  the  belief  that  the  annulose  animal  is 
an  aggregate  of  the  third  order,  the  segments  of  which, 
produced  one  from  another  by  gemmation,  originally 
became  separate,  as  they  still  become  in  the  cestoid 
Entozoa ;  but  that  by  progressive  integration,  or  arrested 
disintegration,  there  resulted  a  type  in  which  many  such 
segments  were  permanently  united  (§§  205-7).  Part  of  the 
evidence  there  assigned,  is  evidence  to  be  here  repeated  in 
illustration  of  the  direct  antagonism  of  Growth  and  Asexual- 
Genesis.  We  saw  how,  among  the  lower  Annelids,  the  string 
of  segments  produced  by  gemmation  presently  divides  trans- 
versely into  two  strings ;  and  how,  in  some  cases,  this  resolu- 
tion of  the  elongating  string  of  segments  into  groups  that 
are  to  form  separate  individuals,  goes  on  so  actively  that  as 
many  as  six  groups  are  found  in  different  stages  of  progress 
to  ultimate  independence — a  fact  implying  a  high  rate  of 
fissiparous  multiplication.  Then  we  saw  that,  in  the  superior 
annulose  types,  distinguished  in  the  mass  by  including  the 
larger  species,  fission  does  not  occur.  The  higher  Annelids 
do  not  propagate  in  this  way ;  there  is  no  known  case  of  new 
individuals  being  so  formed  among  the  Mftriapoda  ;  nor  do 
the  Crustaceans  afford  us  a  single  instance  of  this  primordial 
mode  of  increase.  It  is,  indeed,  true  that  whila 


126-  LAWS    OF    MULTIPLICATION. 

articulate  animals  never  multiply  asexually  after  this  simplest 
method,  and  while  they  are  characterized  in  the  mass  by  the 
cessation  of  agamogenesis  of  every  kind,  there  nevertheless 
occur  in  a  few  of  their  small  species,  those  higher  forms  of 
ctgamogenesis  known  as  parthenogenesis,  pseudo-partheno- 
genesis and  internal  metagenesis  ;  and  that  by  these  some  of 
them  multiply  very  rapidly.  Hereafter  we  shall  find,  in  the 
interpretation  of  these  anomalies,  further  support  for  the 
general  doctrine. 

To  the  above  evidence  has  to  be  added  that  which  the 
Vertebrata  present.  This  may  be  very  briefly  summed  up. 
On  the  one  hand,  this  class,  whether  looked  at  in  the  aggre- 
gate or  in  its  particular  species,  immensely  exceeds  all  other 
classes  in  the  sizes  of  its  individuals ;  and  on  the  other  hand, 
agamogenesis  under  any  form  is  absolutely  unknown  in  it, 

§  337.  Such  are  a  few  leading  facts  serving  to  show  how 
deduction  is  inductively  verified,  in  so  far  as  the  anta- 
gonism between  Growth  and  Asexual  Genesis  is  con- 
cerned. In  whatever  way  we  explain  this  opposition  of 
the  integrative  and  disintegrative  processes,  the  facts  and 
their  implications  remain  the  same.  Indeed  we  need  not 
commit  ourselves  to  any  hypothesis  respecting  the  physical 
causation :  it  suffices  to  recognize  the  results  under  their 
most  general  aspects.  "We  cannot  help  admitting  there  are 
at  work  these  two  antagonist  tendencies  to  aggregation  and 
separation ;  and  we  cannot  help  admitting  that  the  propor- 
tion between  the  aggregative  and  separative  tendencies,  must 
in  each  case  determine  the  relation  between  the  increase  in 
bulk  of  the  individual  and  the  increase  of  the  race  in  number. 

The  antithesis  is  as  manifest  d  posteriori  as  it  is  neces- 
sary d  priori.  While  the  minutest  organisms  multiply 
asexually  in  their  millions ;  while  the  small  compound 
types  next  above  them  thus  multiply  in  their  thousands  ; 
while  larger  and  more  compound  types  thus  multiply  in  their 
hundreds  and  their  tens ;  the  largest  types  do  not  thus 


GROWTH    AND   ASEXUAL    GENESIS.  427 

multiply  at  all.  Conversely,  those  winch  do  not  multiply 
asexually  at  all,  are  a  billion  or  a  million  times  the  size  of 
those  which  thus  multiply  with  greatest  rapidity ;  and  are  a 
thousand  times,  or  a  hundred  times,  or  ten  times  the  size  of 
those  which  thus  multiply  with  less  and  less  rapidity.  With- 
out saying  that  this  inverse  proportion  is  regular,  which,  as  we 
shall  hereafter  see,  it  cannot  be,  we  may  unhesitatingly  assert 
its  average  truth.  That  the  smallest  organisms  habitually 
reproduce  asexually  with  immense  rapidity  ;  that  the  largest 
organisms  never  reproduce  at  all  in  this  manner ;  and  that 
Detween  these  extremes  there  is  a  general  decrease  of  asexual 
reproduction  along  with  an  increase  of  bulk;  are  proposi- 
tions that  admit  of  no  dispute. 


CHAPTER  VI. 

ANTAGONISM   BETWEEN  GROWTH  AND  SEXUAL  GENESIS. 

§  338.  In  so  far  as  it  is  a  process  of  separation,  sexual 
genesis  is  like  asexual  genesis ;  and  is  therefore,  equally  with 
asexual  genesis,  opposed  to  that  aggregation  which  results  in 
growth.  Whether  a  deduction  is  made  from  one  parent  or 
from  two,  whether  it  is  made  from  any  part  of  the  body 
indifferently  or  from  a  specialized  part,  or  whether  it  is  made 
directly  or  indirectly,  it  remains  in  any  case  a  deduction ; 
and  in  proportion  as  it  is  great,  or  frequent,  or  both,  it  must 
restrain  the  increase  of  the  individual. 

Here  we  have  to  group  together  the  leading  illustrations 
of  this  truth.  We  will  take  them  in  the  same  order  as 
before. 

§  339.  The  lowest  vegetal  forms,  or  rather,  we  may  say, 
those  forms  which  we  cannot  class  as  either  distinctly  vegetal 
or  distinctly  animal,  show  us  a  process  of  sexual  multiplica- 
tion that  differs  much  less  from  the  asexual  process  than  in 
the  higher  forms.  The  common  character  which  distinguishes 
sexual  from  asexual  genesis,  is  that  the  mass  of  protoplasm 
whence  a  new  generation  is  to  arise,  has  been  produced  by  the 
union  of  two  portions  of  matter  that  were  before  more  widely 
separated.  I  use  this  general  expression,  because  among  tho 
simplest  Algce,  this  is  not  invariably  matter  supplied  by 
different  individuals :  certain  Dlatomaccce  exhibit  within  a 
single  cell,  the  formation  of  a  sporangium  by  a  drawing 


GROWTH    AND    SEXUAL    GENESIS.  429 

together  of  the  opposite  halves  of  the  endochrome  info  a 
ball.  Mostly,  however,  sporangia  are  products  of  conjuga- 
tion. The  endochromes  of  two  cells  unite  to  form  the  germ- 
mass;  and  these  conjugating  cells  may  be  either  entirely 
independent,  as  in  many  Desmidiacece  and  in  the  PalrncUce;  or 
they  may  be  two  of  the  adjacent  cells  forming  a  thread,  as  in. 
some  Conjugate ;  or  they  may  be  cells  belonging  to  adjacent 
threads,  as  in  Zygne'ma.  But  whether  it  is  originated  by  a 
single  parent-cell,  or  by  two  parent-cells,  the  sporangium, 
after  remaining  quiescent  until  there  recur  the  fit  conditions 
for  growth,  breaks  up  into  a  multitude  of  spores,  each  of  which 
produces  an  individual  that  multiplies  asexuully  ;  and  the  fact 
here  to  be  noted  is,  that  as  the  entire  contents  of  the  parent- 
cells  unite  to  form  the  sporangium,  their  individualities  are  lost 
in  the  germs  of  a  new  generation.  In  these  minute  simple 
types,  sexual  propagation  just  as  completely  sacrifices  the  life 
of  the  parent  or  parents,  as  does  that  form  of  asexual  propa- 
gation in  which  the  endochrome  resolves  itself  directly  into 
zoospores.  And  in  the  one  case  as  in  the  other,  this  sacrifice 
is  the  concomitant  of  a  prodigious  fertility.  Slightly 

in  advance  of  this,  but  still  showing  us  an  almost  equal  loss 
of  parental  life  in  the  lives  of  offspring,  is  the  process  seen  in 
such  unicellular  Algce  as  ITydrogastrum,  and  in  minute  Fungi 
of  the  same  degree  of  composition.  These  exhibit  a  relatively- 
enormous  development  of  the  spore-producing  part,  and  an 
almost  entire  absorption  of  the  parental  substance  into  it. 
As  evidence  of  the  resulting  powers  of  multiplication,  we 
have  but  to  remember  that  the  spread  of  mould  over  stale 
food,  the  rapid  destruction  of  crops  by  mildew,  and  other 
kindred  occurrences,  are  made  possible  by  the  incalculably 
numerous  spores  thus  generated  and  universally  dispersed. 

Plants  a  degree  higher  in  composition,  supply  a  parallel 
series  of  illustrations.  We  have  among  the  larger  Fungi,  in 
which  the  reproductive  apparatus  is  relatively  so  enormous  as 
to  constitute  the  ostensible  plant,  a  similar  subordination  of 
the  individual  to  the  race,  and  a  similarly- immense  fertility. 


430  LAWS   OF    MULTiriJCATION. 

Thus,  as  quoted  by  Dr.  Carpenter,  Fries  says — "  in  a  single 
individual  of  Reticularia  maxima,  I  have  counted  (calculated?) 
10,000,000  sporules."  It  needs  but  to  note  the  clouds  of 
particles,  so  minute  as  to  look  like  smoke,  which  ripe  puff- 
balls  give  off  when  they  are  burst,  and  then  to  remember 
that  each  particle  is  a  potential  fungus,  to  be  impressed  with 
the  almost  inconceivable  powers  of  propagation  which  these 
plants  possess.  The  Lichens,  too,  furnish  examples. 

Though  they  are  nothing  like  so  prolific  as  the  Fungi  (the 
difference  yielding,  as  we  shall  hereafter  see,  further  support 
to  the  general  argument),  yet  there,  is  a  great  production  of 
germs,  and  a  proportionate  sacrifice  of  the  parental  indi- 
viduality. Considerable  areas  of  the  frond  here  and  there 
develop  into  apothecia  and  spcnnagonia,  which  resolve  them- 
selves into  sperm-cells  and  germ-cells.  Some  con- 
trasts presented  by  the  higher  Algce  may  also  be  named  as 
exemplifying  the  inverse  proportion  between  the  size  of  the 
individual  and  the  extent  of  the  generative  structures.  While 
in  the  smaller  kinds  relatively  large  portions  of  the  fronds  are 
transformed  into  reproductive  elements,  in  the  larger  kinds 
these  portions  are  relatively  small :  instance  the  Macrocystis 
pyrifera,  a  gigantic  sea-weed,  which  sometimes  attains  a 
length  of  1,500  feet,  of  which  Dr.  Carpenter  remarks  — 
"  This  development  of  the  nutritive  surface  takes  place  at 
the  expense  of  the  fructifying  apparatus,  which  is  here  quite 
subordinate." 

"When  we  turn  to  vegetal  aggregates  of  the  third  order  of 
composition,  facts  having  the  same  meaning  are  conspicuous. 
On  the  average  these  higher  plants  are  far  larger  than 
plants  of  a  lower  degree  of  composition ;  and  on  the  average 
their  rates  of  sexual  reproduction  are  far  less.  Similarly  if, 
among  Acrogens,  Endogens,  and  Exogens,  we  compare  the 
smaller  types  with  the  larger,  \ve  find  them  proportionately 
more  prolific.  This  is  not  manifest  if  we  simply  calculate 
the  number  of  seeds  ripened  by  an  individual  in  a  single 
season  :  but  it  becomes  manifest  if  we  take  into  account  the 


GROWTH    AND    SEXUAL   GENESIS.  4'U 

further  factor  which  here  complicates  the  result — the  age  at 
which  sexual  genesis  commences.  The  smaller  Phsenogama 
are  mostly  either  annuals,  or  perennials  that  die  down 
annually ;  and  seeding  as  they  do  annually  before  their 
deaths,  or  the  deaths  of  their  reproductive  parts,  it  results 
that  in  the  course  of  a  year,  each  gives  origin  to  a  multitude 
of  potential  plants,  of  which  every  one  may  the  next  year,  if 
preserved,  give  origin  to  an  equal  multitude.  Supposing  but 
a  hundred  offspring  to  be  produced  the  first  year,  ten 
thousand  may  be  produced  in  the  second  year,  a  million  in 
the  third,  a  hundred  millions  in  the  fourth.  Meanwhile, 
what  has  been  the  possible  multiplication  of  a  large  Phoe- 
nogam?  While  its  small  congener  has  been  seeding  and 
dying,  and  leaving  multitudinous  progeny  to  seed  and  die,  it 
has  simply  been  growing ;  and  may  so  continue  to  grow  for 
ten  or  a  dozen  years  without  bearing  fruit.  Before  a  Cocoa- 
nut  tree  has  ripened  its  first  cluster  of  nuts,  the  descendants 
of  a  wheat  plant,  supposing  them  all  to  survive  and  multiply, 
will  have  become  numerous  enough  to  occupy  the  whole 
surface  of  the  Earth.  So  that  though,  when  it  begins  to 
bear,  a  tree  may  annually  shed  as  many  seeds  as  a  herb,  yet 
in  consequence  of  this  delay  in  bearing,  its  fertility  is  incom- 
parably less ;  and  its  relatively-small  fertility  becomes  still 
further  reduced  where,  as  in  Lodoicea  Sechellarum,  the  seeds 
take  two  years  from  the  date  of  fertilization  to  the  date  of 
germination. 

§  340.  Some  observers  state  that  in  certain  Protozoa  there 
occurs  a  process  of  conjugation  akin  to  that  which  the 
Protopliyia  exhibit — a  coalescence  of  the  substance  of  two 
individuals  to  form  a  germ-mass.  This  has  been  alleged 
more  especially  of  Actinophrys.  The  statement  is  question- 
able ;  but  if  proved  true,  then  of  the  minute  forms  that 
appear  to  be  more  animal  than  vegetal  in  their  characters, 
some  have  a  mode  of  sexual  multiplication  by  which  the 
parents  are  sacrificed  bodily  in  the  production  of  a  new 
VOL.  IT.  19 


432  LAWS    OF    MULTIPLICATION. 

generation.  A  modified  mode,  apparently  not  fatal  to  the 
parents,  lias  been  observed  in  certain  of  the  more  developed 
Infusoria.  Our  knowledge  of  these  microscopic  types  is, 
however,  so  rudimentary  that  evidence  derived  from  them 
must  be  taken  with  a  qualification. 

Among  small  animal  aggregates  of  the  second  order,  the 
first  to  be  considered  are  of  course  the  Ccelenterata.  A  Hydra 
occasionally  devotes  a  large  part  of  its  substance  to  sexual 
genesis.  In  the  walls  of  its  body  groups  of  ova,  or  sperma- 
tozoa, or  both,  take  their  rise;  and  develop  into  masses 
greatly  distorting  the  creature's  form,  and  leaving  it  greatly 
diminished  when  they  escape.  Here,  however,  gamogenesis  is 
obviously  supplementary  to  agamogenesis — the  immensely 
rapid  multiplication  by  budding  continues  as  long  as  food  is 
abundant  and  warmth  sufficient,  and  is  replaced  by  gamo- 
genesis only  at  the  close  of  the  season.  A  better 
example  of  the  relation  between  small  size  and  active  gamo- 
genesis is  supplied  by  the  Planar  ia,  which  does  not  multiply 
asexually  with  so  much  rapidity.  The  generative  system  is 
here  enormous.  Ova  are  developed  all  through  the  body, 
occupying  everywhere  the  interspaces  of  the  assimilative 
system ;  so  that  the  animal  may  be  said  to  consist  of  a  part 
that  absorbs  nutriment  and  a  part  that  transforms  that  nutri- 
ment into  sperm-cells  and  germ-cells.  Even  saying  nothing 
of  the  probably- early  maturity  of  these  animals,  and  there- 
fore fuequent  repetition  of  sexual  multiplication,  it  is  clear 
that  their  fertilit}'  must  be  very  great. 

The  Annulosa,  including  among  them  the  inferior  kindred 
types,  have  habits  and  conditions  of  life  so  various  that  only 
the  broadest  contrasts  can  be  instanced  in  support  of  the  pro- 
position before  us.  Of  the  microscopic  forms  belonging  to 
this  sub-kingdom,  the  Hot  if  era  may  be  named  as  having, 
along  with  small  bulk,  a  great  rate  of  sexual  increase.  Hycla- 
tina  senta  "  is  capable  of  a  four-fold  propagation  every  twenty- 
four  or  thirty-hours,  bringing  forth  in  this  time  four  ova, 
which  grow  from  the  embryo  to  maturity,  and  exclude  their 


3ROWTH   AND   SEXUAL   GENESIS.  43fi 

fertile  ova  in  the  same  period.  The  same  individual,  pro- 
ducing in  ten  days  forty  eggs,  developed  with  the  rapidity 
above  cited,  this  rate,  raised  to  the  tenth  power,  gives  one 
million  of  individuals  from  one  parent,  on  the  eleventh  day 
four  millions,  and  on  the  twelfth  day  sixteen  millions,  and  so 
on."  Ascending  from  this  extreme,  the  differences 

of  organization  and  activity  greatly  complicate  the  inverse 
variation  of  fertility  and  bulk.  Bearing  in  mind,  how- 
ever, that  the  rate  of  multiplication  depends  much  less  on  the 
number  of  each  brood  than  on  the  quickness  with  which 
maturity  is  reached  and  a  new  generation  commenced,  it  will 
be  obvious  that  though  Annelids  produce  great  numbers  of 
ova,  yet  as  they  do  this  at  comparatively  long  intervals,  their 
rates  of  increase  fall  immensely  below  that  just  instanced  in 
the  Rotifers.  And  when  at  the  other  extreme  we  come  to 
the  large  articulate  animals,  such  as  the  Crab  and  the  Lobster, 
the  further  diminution  of  fertility  is  seen  in  the  still  longer 
delay  that  occurs  before  each  new  generation  begins  to  re- 
produce. 

Perhaps  the  best  examples  are  supplied  by  vertebrate 
animals,  and  especially  those  that  are  most  familiar  to  us. 
Comparisons  between  Fishes  are  unsatisfactory,  because  of 
our  ignorance  of  their  histories.  In  some  cases  Fishes  equal 
in  bulk  produce  widely  different  numbers  of  eggs ;  as  the 
Cod  which  spawns  a  million  at  once,  and  the  Salmon  by 
which  nothing  like  so  great  a  number  is  spawned.  But  then 
the  eggs  are  very  unlike  in  size  ;  and  if  the  ovaria  of  the  two 
fishes  be  compared,  the  difference  between  their  masses  is 
comparatively  moderate.  There  are,  indeed,  contrasts  which 
seem  at  variance  with  the  alleged  relation ;  as  that  between 
the  Cod  and  the  Stickleback,  which,  though  so  much  smaller, 
produces  fewer  ova.  The  Stickleback's  ova,  however,  are 
relatively  large ;  and  their  total  bulk  bears  as  great  a  ratio  to 
the  bulk  of  the  Stickleback  as  does  the  bulk  of  the  Cod's  ova 
to  that  of  the  Cod.  Moreover,  if,  as  is  not  improbable,  the 
reproductive  age  is  arrived  at  earlier  by  the  Stickleback  than 


434  LAWS   OF    MULTIPLICATION. 

by  the  Cod,  the  fertility  of  the  species  may  oe  greater  not- 
withstanding the  smaller  number  produced  by  each  indi- 
vidual. Evidence  that  admits  of  being  tolerably 
well  disentangled  is  furnished  by  Birds.  They  differ  but 
little  in  their  grades  of  organization ;  and  the  habits  of  life 
throughout  extensive  groups  of  them  are  so  similar,  that 
comparisons  may  be  fairly  made.  It  is  true  that,  as  hereafter 
to  be  shown,  the  differences  of  expenditure  which  differences 
of  bulk  entail,  have  doubtless  much  to  do  with  the  differences 
of  fertility.  But  we  may  set  down  under  the  present  head 
some  of  those  cases  in  which  the  activity,  being  relatively 
slight,  does  not  greatly  interfere  with  the  relation  we  are 
considering;  and  may  note  that  among  such  birds  having 
similarly  slight  activities,  the  small  produce  more  eggs  than 
the  large,  and  eggs  that  bear  in  their  total  mass  a  greater 
ratio  to  the  mass  of  the  parent.  Consider,  for  example,  the 
gallinaceous  birds ;  which  are  like  one  another  and  unlike 
birds  of  most  other  groups  in  flying  comparatively  little. 
Taking  first  the  wild  members  of  this  order,  which  rarely  breed 
more  than  once  in  a  season,  we  find  that  the  Pheasant  has 
from  6  to  10  eggs,  the  Black-cock  from  5  to  10,  the  Grouse 
8  to  12,  the  Partridge  10  to  15,  the  Quail  still  more,  some- 
times reaching  20.  Here  the  only  exception  to  the  relation 
between  decreasing  bulk  and  increasing  number  of  eggs, 
occurs  in  the  cases  of  the  Pheasant  and  the  Black-cock ;  and 
it  is  to  be  remembered,  in  explanation,  that  the  Pheasant 
inhabits  a  warmer  region  and  is  better  fed — often  artificially. 
If  we  pass  to  domesticated  genera  of  the  same  order,  we 
meet  with  parallel  differences.  From  the  numbers  of  eggs 
laid,  little  can  be  inferred  ;  for  under  the  favourable  con- 
ditions artificially  maintained,  the  laying  is  carried  on  inde- 
finitely. But  though  in  the  sizes  of  their  broods  the  Turkey 
and  the  Fowl  do  not  greatly  differ,  the  Fowl  begins  breeding 
at  a  much  earlier  age  than  the  Turkey,  and  produces 
broods  more  frequently :  a  considerably  higher  rate  of 
multiplication  being  the  result.  Now  these  contrasts 


GROWTH   AND   SEXUAL   GENESIS.  435 

among  domestic  creatures  that  are  similarly  conditioned, 
and  closely -allied  by  constitution,  may  be  held  to  show, 
more  clearly  than  most  other  contrasts,  the  inverse  varia- 
tion between  bulk  and  sexual  genesis ;  since  here  the 
cost  of  activity  is  diminished  to  a  comparatively  small 
amount.  There  is  little  expenditure  in  flight — sometimes 
almost  none ;  and  the  expenditure  in  walking  about  is 
not  great:  there  is  more  of  standing  than  of  actual 
movement.  It  is  true  that  young  Turkeys  commence 
their  existences  as  larger  masses  than  chickens ;  but  it  is 
tolerably  manifest  that  the  total  weight  of  the  eggs  produced 
by  a  Turkey  during  each  season,  bears  a  less  ratio  to  the 
Turkey's  weight,  than  the  total  weight  of  the  eggs  which  a 
Hen  produces  during  each  season,  bears  to  the  Hen's  weight ; 
and  this  is  the  fairest  way  of  making  the  comparison.  The 
comparison  so  made  shows  a  greater  difference  than  appears 
likely  to  be  due  to  the  different  costs  of  locomotion;  con- 
sidering the  inertness  of  the  creatures.  Remembering  that 
the  assimilating  surface  increases  only  as  the  squares  of  the 
dimensions,  while  the  mass  of  the  fabric  to  be  built  up  by  the 
absorbed  nutriment  increases  as  the  cubes  of  the  dimensions, 
it  will  be  seen  that  the  expense  of  growth  becomes  relatively 
greater  with  each  increment  of  size ;  and  that  hence,  of  two 
similar  creatures  commencing  life  with  different  sizes,  the 
larger  one  in  reaching  its  superior  adult  bulk,  will  do  this  at 
a  more  than  proportionate  expense  ;  and  so  will  either  be 
delayed  in  commencing  its  reproduction,  or  will  have  a 
diminished  reserve  for  reproduction,  or  both.  Other  orders 
of  Birds,  active  in  their  habits,  show  more  markedly  the  con- 
nexion between  augmenting  mass  and  declining  fertility. 
But  in  them  the  increasing  cost  of  locomotion  becomes  an 
important,  and  probably  the  most  important,  factor.  The 
evidence  they  furnish  will  therefore  come  better  under 
another  head.  Contrasts  among  Mammals,  like 

those  which  Birds  present,  have  their  meanings  obscured  by 
inequalities  of  the  expenditure  for  motion.  The  smaller 


i3G  LAWS   OF    MULTIPLICATION". 

fertility  which  habitually  accompanies  greater  bulk,  must 
in  all  cases  be  partly  ascribed  to  this.  Still,  it  may  be 
well  if  we  briefly  note,  for  as  much  as  they  are  worth, 
the  broader  contrasts.  While  a  large  Mammal  bears  but 
a  single  young  one  at  a  time,  is  several  years  before  it 
commences  doing  this,  and  then  repeats  the  reproduction  at 
long  intervals ;  we  find,  as  we  descend  to  the  smaller  mem- 
bers of  the  class,  a  very  early  commencement  of  breeding,  an 
increasing  number  at  a  birth,  reaching  in  small  Rodents  to 
10  or  even  more,  and  a  much  more  frequent  recurrence  of 
broods:  the  combined  result  being  a  relatively  prodigious 
fertility.  If  a  specific  comparison  be  desired  between 
Mammals  that  are  similar  in  constitution,  in  food,  in  con- 
ditions of  life,  and  all  other  things  but  size,  the  Deer-tribe 
supplies  it.  "While  the  large  Red-deer  has  but  one  at  a 
birth,  the  small  Roe-deer  has  two  at  a  birth. 

§  341.  The  antagonism  between  growth  and  sexual  genesis, 
visible  in  these  general  contrasts,  may  also  be  traced  in  the 
history  of  each  plant  and  animal.  So  familiar  is  the  fact 
that  sexual  genesis  does  not  occur  early  in  life,  and  in  all 
organisms  which  expend  much  begins  only  when  the  limit  of 
size  is  nearly  reached,  that  we  do  not  sufficiently  note  its 
significance.  It  is  a  general  physiological  truth,  however, 
that  while  the  'building-up  of  the  individual  is  going  on 
rapidly,  the  reproductive  organs  remain  imperfectly  developed 
and  inactive ;  and  that  the  commencement  of  reproduction 
at  once  indicates  a  declining  rate  of  growth,  and  becomes  a 
cause  of  arresting  growth.  As  was  shown  in  §  78,  the  ex- 
ceptions to  this  rule  are  found  where  the  limit  of  growth  is 
indefinite  ;  either  because  the  organism  expends  little  or 
nothing  in  action,  or  expends  in  action  so  moderate  an 
amount  that  the  supply  of  nutriment  is  never  equilibrated 
by  its  expenditure. 

We  will  pass  ever  the  inferior  plants,  and  limiting  our- 
selves to  Phaenogams,  will  not  dwell  on  the  less  conspicu- 


OUOWTH    AM)   SEXUAL    GENESIS.  437 

ens  evidence  which  the  smaller  types  present.  A  few  cases 
such  as  gardens  supply  will  serve.  All  know  that  a  Pear- 
tree  continues  to  increase  in  size  for  years  before  it  begins  to 
bear ;  and  that,  producing  but  few  pears  at  first,  it  is  long 
before  it  fruits  abundantly.  A  young  Mulberry,  branch- 
ing out  luxuriantly  season  after  season,  but  covered 
•«  ith  nothing  but  leaves,  at  length  blossoms  sparingly,  and 
sets  some  small  and  imperfect  berries,  which  it  drops  while 
they  are  green ;  and  it  makes  these  futile  attempts  time  after 
time  before  it  succeeds  in  ripening  any  seeds.  But  these 
multi-axial  plants,  or  aggregates  of  individuals  some  of 
which  continue  to  grow  while  others  become  arrested  and 
transformed  into  seed-bearers,  show  us  the  relation  less  de- 
finitely than  certain  plants  that  are  substantially,  if  not 
literally,  uni-axial.  Of  these  the  Cocoa-nut  may  be  in- 
stanced. For  some  years  it  goes  on  shooting  up  without 
making  any  sign  of  becoming  fertile.  About  the  sixth  year 
it  flowers  ;  but  the  flowers  wither  without  result.  In  the 
seventh  year  it  flowers  and  produces  a  few  nuts  ;  but  these 
prove  abortive  and  drop.  In  the  eighth  year  it  ripens  a 
moderate  number  of  nuts;  and  afterwards  increases  the 
number  until,  in  the  tenth  year,  it  comes  into  full  bearing. 
Meanwhile,  from  the  time  of  its  first  flowering  its  growth 
begins  to  diminish,  and  goes  on  diminishing  till  the  tenth 
year,  when  it  ceases.  Here  we  see  the  antagonism  between 
growth  and  sexual  genesis  under  both  its  aspects — see  a 
struggle  between  self-evolution  and  race- evolution,  in  which 
the  first  for  a  time  overcomes  the  last,  and  the  last  ultimately 
overcomes  the  first.  The  continued  aggrandisement  of  the 
parent-individual  makes  abortive  for  two  seasons  the  tendency 
to  produce  new  individuals;  and  the  tendency  to  produce 
new  individuals,  becoming  more  decided,  stops  any  furthor 
aggrandisement  of  the  parent-individual. 

Parallel  illustrations  occur  in  the  animal  kingdom.  The 
eggs  laid  by  a  pullet  are  relatively  small  and  few.  Similarly, 
it  is  alleged  that,  as  a  general  rule,  "  a  bitch  has  fewer 


138  LAWS   OF    MULTII'LICATIOX 

puppies  at  first,  than  afterwards."  According  to  Burdach, 
as  quoted  by  Dr.  Duncan,  "  the  elk,  the  bear,  &c.,  have  at 
first  only  a  single  young  one,  then  they  come  to  have  most 
frequently  two,  and  at  last  again  only  one.  The  young 
hamster  produces  only  from  three  to  six  young  ones,  whilst 
that  of  a  more  advanced  age  produces  from  eight  to  sixteen. 
The  same  is  true  of  the  pig."  It  is  remarked  by  Buffon  that 
when  a  sow  of  less  than  a  year  old  has  young,  the  number  of 
the  litter  is  small,  and  its  members  are  feeble  and  even  im- 
perfect. Here  we  have  evidence  that  in  animals  growth 
checks  sexual  genesis.  And  then,  conversely,  we  have 
evidence  that  sexual  genesis  checks  growth.  It  is  well 
known  to  breeders  that  if  a  filly  is  allowed  to  bear  a  foal, 
she  is  thereby  prevented  from  reaching  her  proper  size.  And 
a  like  loss  of  perfection  as  an  individual,  is  suffered  by  a 
cow  that  breeds  too  early. 

§  342.  Notwithstanding  the  way  in  which  the  inverse 
variation  of  growth  and  sexual  genesis  is  complicated  with 
other  relations,  its  existence  is  thus,  I  think,  sufficiently  mani- 
fest. Individually,  many  of  the  foregoing  instances  are  open 
to  criticism,  and  have  to  be  taken  with  qualifications  ;  but 
when  looked  at  in  the  mass,  their  meaning  is  beyond  doubt. 
Comparisons  between  the  largest  with  the  smallest  types, 
whether  vegetal  or  animal,  yield  results  that  are  unmis- 
takeable.  On  the  one  hand,  remembering  the  fact  that 
during  its  centuries  of  life  an  Oak  does  not  produce  as  many 
acorns  as  a  Fungus  does  spores  in  a  single  night,  we  see  that 
the  Fungus  has  a  fertility  exceeding  that  of  the  Oak  in  a  de- 
gree literally  beyond  our  powers  of  calculation  or  imagina- 
tion. "When,  on  the  other  hand,  taking  a  microscopic 
protophyte  which  has  millions  of  descendants  in  a  few  days, 
we  ask  how  many  such  would  be  required  to  build  up  the 
forest  tree  that  is  years  before  it  drops  a  seed,  we  are  met 
by  a  parallel  difficulty  in  conceiving  the  number,  if  not  in 
setting  it  down.  Similarly,  if  we  turn  from  the  minute  and 


GROWTH   AND   SEXUAL   GENESIS.  439 

prodigiously-fertile  Rotifer,  to  the  Elephant,  which  approaches 
thirty  years  before  it  bears  a  solitary  young  one,  we  find  the 
connexions  between  small  size  and  great  fertility  and  between 
great  size  and  small  fertility,  too  intensely  marked  to  be 
much  disguised  by  the  perturbing  relations  that  have  been 
indicated.  Finally,  as  this  induction,  reached  by  a  survey  of 
organisms  in  general,  is  verified  by  observations  on  the  rela- 
lioa  between  decreasing  growth  and  commencing  reproduc- 
tion in  individual  organisms,  we  may,  I  think,  consider  the 
alleged  antagonism  as  proved.* 

*  When,  after  having  held  for  some  years  the  general  doctrine  elaborated  in 
these  chapters,  I  agreed,  early  in  1852,  to  prepare  an  outline  of  it  for  the  West- 
minster Review,  I  consulted,  among  other  works,  the  just-issued  third  editicm 
of  Dr.  Carpenter's  Principles  of  Physiology,  General  and  Comparative — seeking 
in  it  for  facts  illustrating  the  different  degrees  of  fertility  of  different  organisms. 
I  met  with  a  passage,  quoted  above  in  §  339,  which  seemed  tacitly  to  assert 
that  individual  aggrandizement  is  at  variance  with  the  propagation  of  the  race  ; 
but  nowhere  found  a  distinct  enunciation  of  this  truth.  I  did  not  then  read 
the  Chapter  entitled  "General  View  of  the  Functions,"  which  held  out 
no  promise  of  such  evidence  as  I  was  looking  for.  But  on  siuce  referring  to 
this  chapter,  I  discovered  in  it  the  definite  statement  that — "there  is  a  certain 
degree  of  antagonism  between  the  Nutritive  and  Reproductive  functions,  the 
one  being  executed  at  the  expense  of  the  other.  The  reproductive  apparatus 
derives  the  materials  of  its  operations  through  the  nutritive  system,  and 
is  entirely  dependent  upon  it  for  the  continuance  of  its  function.  If,  there- 
fore, it  be  in  a  state  of  excessive  activity,  it  will  necessarily  draw  off  from  tho 
individual  fabric  some  portion  of  the  aliment  destined  for  its  maintenance. 
It  may  be  universally  observed  that,  when  the  nutritive  functions  are 
particularly  active  in  supporting  the  individual,  tho  reproductive  system  is  in 
a  corresponding  degree  undeveloped, — and  vice  versd." —Principles  of  Phy- 
fiology,  General  and  Comparative,  Third  Edition,  1851,  p.  592. 


CHAPTER  VII. 


THE    ANTAGONISM    BETWEEN   DEVELOPMENT   AND   GENESIS, 
ASEXUAL  AND  SEXUAL. 


§  343.  By  Development,  as  here  to  be  dealt  with  apart 
from  Growth,  is  meant  increase  of  structure  as  distinguished 
from  increase  of  mass.  As  was  pointed  out  in  §  50,  this  is 
the  biological  definition  of  the  word.  In  the  following 
sections,  then,  we  have  to  note  how  complexity  of  organiza- 
tion is  hindered  by  reproductive  activity,  and  conversely. 

This  relation  partially  coincides  with  that  which  we  have 
just  contemplated;  for,  as  was  shown  in  §44,  degree  of 
growth  is  to  a  considerable  extent  dependent  on  degree  of 
organization.  But  while  the  antagonism  to  be  illustrated  in 
this  chapter,  is  much  entangled  with  that  illustrated  in  the 
last  chapter,  it  may  be  so  far  separated  as  to  be  identified  as 
an  additional  antagonism. 

Besides  the  direct  opposition  between  that  continual  dis- 
integration which  rapid  genesis  implies,  and  the  fulfilment  of 
that  pre- requisite  to  extensive  organization — the  formation  of 
an  extensive  aggregate,  there  is  an  indirect  opposition  which 
we  may  recognize  under  several  aspects.  The  change 

from  homogeneity  to  heterogeneity  takes  time ;  and  time  taken 
in  transforming  a  relatively-structureless  mass  into  a  de 
veloped  individual,  delays  the  period  of  reproduction.  Usually 
this  time  is  merged  in  that  taken  for  growth;  but  certain 
cases  of  metamorphosis  show  us  the  one  separate  from  the 


DEVELOPMENT   AND    GENESIS.  441 

flther.  An  insect,  passing  from  its  lowly- organized  cater- 
pillar-stage into  that  of  chrysalis,  is  afterwards  a  week,  a  fort- 
night, or  a  longer  period  in  completing  its  structure :  the  re- 
commencement of  genesis  being  by  so  much  postponed,  and 
the  rate  of  multiplication  therefore  diminished.  Further,  that 
re- arrangement  of  substance  which  development  implies,  en- 
tails expenditure.  The  chrysalis  loses  weight  in  the  course 
of  its  transformation ;  and  that  its  loss  is  not  loss  of  water 
only,  may  be  inferred  from  the  fact  that  it  respires,  and  that 
respiration  indicates  consumption.  Clearly  the  matter  con- 
sumed, is,  other  things  equal,  a  deduction  from  the  surplus 
that  may  go  to  reproduction.  Yet  again,  the 

more  widely  and  completely  an  organic  mass  becomes  diffe- 
rentiated, the  smaller  the  portion  of  it  which  retains  the  re- 
latively-undifferentiated  state  that  admits  of  being  moulded 
into  new  individuals,  or  the  germs  of  them.  Protoplasm 
which  has  become  specialized  tissue,  cannot  be  again 
generalized,  and  afterwards  transformed  into  something  else ; 
and  hence  the  progress  of  structure  in  an  organism,  by 
diminishing  the  unstructured  part,  diminishes  the  amount 
available  for  making  offspring. 

It  is  true  that  higher  structure,  like  greater  growth,  may 
insure  to  a  species  advantages  that  eventually  further  its  mul- 
tiplication— may  give  it  access  to  larger  supplies  of  food,  or 
enable  it  to  obtain  food  more  economically ;  and  we  shall 
hereafter  see  how  the  inverse  variation  we  are  considering  is 
thus  qualified.  But  here  we  are  concerned  only  with  the 
necessary  and  direct  effects ;  not  with  those  that  are  con- 
tingent and  remote.  These  necessary  and  direct  effects  wo 
will  now  look  at  as  exemplified. 

§  344.  Speaking  generally,  the  simpler  plants  propagate 
both  sexually  and  asexually ;  and,  speaking  comparatively, 
1  he  complex  plants  propagate  only  sexually:  their  asexual 
propagation  is  usually  incomplete — produces  a  united  aggre- 
'gate  of  individuals  instead  of  numerous  distinct  individuals. 


442  LAWS    Ol'    MULTIPLICATION. 

The  Protophytes  that  perpetually  subdivide,  the  merely- 
cellular  AlgcB  that  shed  their  tetraspores,  the  Acrogens  that 
spontaneously  separate  their  fronds  and  drop  their  gemma), 
show  us  an  extra  mode  of  multiplication  which,  among  flower- 
ing plants,  is  exceptional.  This  extra  mode  of  multiplication 
among  these  simpler  plants,  is  made  easy  by  their  low  de- 
velopment. Tetraspores  arise  only  where  the  frond  consists 
of  untransformed  cells ;  gemmce  bud  out  and  drop  off  only 
where  the  tissue  is  comparatively  homogeneous. 

Should  it  be  said  that  this  ia  but  another  aspect  of  the 
antagonism  already  set  forth,  since  these  undeveloped  forms 
are  also  the  smaller  forms ;  the  reply  is  that  though  in  part 
true,  this  is  not  wholly  true.  Various  marine  Algce  which 
propagate  asexually,  are  larger  than  some  Phaenogams  which 
do  not  thus  propagate.  The  objection  that  difference  of 
medium  vitiates  this  comparison,  is  met  by  the  fact  that  it  is 
the  same  among  land -plants  themselves.  Sundry  of  the 
lowly- organized  Liverworts  that  are  habitually  gemmiparous, 
exceed  in  size  many  flowering  plants.  And  the  Ferns  show 
us  agamic  multiplication  occurring  in  plants  which,  while 
they  are  inferior  in  complexity  of  structure,  are  superior  in 
bulk  to  a  great  proportion  of  annual  Endogens  and  Exogens. 

§  345.  In  the  ability  of  the  lowly-organized,  or  almost 
unorganized,  sarcode  of  a  Sponge,  to  transform  itself  into 
multitudes  of  gemmules,  we  have  an  instance  of  this  same 
direct  relation  in  the  animal  kingdom.  Moreover,  the 
instance  yields  very  distinct  proof  of  an  antagonism  between 
development  and  genesis,  independent  of  the  antagonism 
between  growth  and  genesis ;  for  the  Sponge  which  thus 
multiplies  itself  asexually,  as  well  as  sexually,  is  far  larger 
than  hosts  of  more  complex  animals  which  do  not  multiply 
asexually. 

Once  again  may  be  cited  the  creature  so  often  brought  in 
evidence,  the  Hydra,  as  showing  us  how  rapidity  of  agamic 
propagation  is  associated  with  inferiority  of  structure.  Ita 


DEVELOPMENT   AND   GENESIS.  443 

power  to  produce  young  ones  from  nearly  all  parts  of  its 
body,  is  due  to  the  comparative  homogeneity  of  its  body.  In 
kindred  but  more-organized  types,  the  gemmiparity  is 
greatly  restricted,  or  disappears.  Among  the  free-swimming 
Hydrazoa,  multiplication  by  budding,  when  it  occurs  at  all, 
occurs  only  at  special  places.  That  increase  of  structure 
apart  from  increase  of  size,  is  here  a  cause  of  declining  agamo- 
genesis,  we  may  see  in  the  contrast  between  the  simple  and 
the  compound  Hydroida ;  which  last,  along  with  more- 
differentiated  tissues,  show  us  a  gemmation  which  does  not 
go  on  all  over  the  body  of  each  polype,  and  much  of  which 
does  not  end  in  separation. 

It  is,  however,  among  the  Annulosa  that  progressing 
organization  is  most  conspicuously  operative  in  diminishing 
agamogenesis.  The  segments  or  "  somites  "  that  compose  an 
animal  belonging  to  this  class,  are  primordially  alike ;  and, 
as  before  argued  (§§  205-7),  are  probably  the  homologues  of 
what  were  originally  independent  individuals.  The  progress 
from  the  lower  to  the  higher  types  of  the  class,  is  at  once  a 
progress  towards  types  in  which  the  strings  of  segments  cease 
to  undergo  subdivision,  and  towards  types  in  which  the  seg- 
ments, no  longer  alike  in  their  structures  and  functions,  have 
become  physiologically  integrated  or  mutually  dependent. 
Already  this  group  of  cases  has  been  named  as  illustrating 
the  antagonism  between  growth  and  asexual  genesis ;  but  it 
is  proper  also  to  name  it  here ;  since,  on  the  one  hand,  the 
greater  size  due  to  the  ceasing  of  fission,  is  made  possible  only 
by  the  specialization  of  parts  and  the  development  of  a  co- 
ordinating apparatus  to  combine  their  actions,  and  since,  on 
the  other  hand,  specialization  and  co-ordination  can  advance 
only  in  proportion  as  fission  ceases. 

§  346.  The  inverse  variation  of  development  and  sexual 
genesis  is  by  no  means  easy  to  follow.  One  or  two  facts  indi- 
cative of  it  may,  however,  be  named. 

Phuenogams  that  have  but  little   supporting  tissue   may 


144  LAWS    OF    MULTIPLICATION*. 

fairly  be  classed  as  structurally  inferior  to  those  provided  with 
stems  formed  of  woody  fibres ;  for  these  imply  additional  dif- 
ferentiations, and  constitute  wider  departures  from  the  primi- 
tive type  of  vegetal  tissue.  That  the  concomitant  of  this 
higher  organization  is  a  slower  gamogenesis,  scarcely  needs 
pointing  out.  While  the  herbaceous  annual  is  blossoming 
and  ripening  seed,  the  young  tree  is  transforming  its  ori- 
ginally-succulent axis  into  dense  fibrous  substance  ;  and  year 
by  year  the  young  tree  expends  in  doing  the  like,  nutriment 
which  successive  generations  of  the  annual  expend  in  fruit. 
Here  the  inverse  relation  is  between  sexual  reproduction  and 
complexity,  and  not  between  sexual  reproduction  and  bulk 
seeing  that  besides  seeding,  the  annual  often  grows  to  a  size 
greater  than  that  reached  by  the  young  infertile  tree  in 
several  years. 

Proof  of  the  antagonism  between  complexity  and  gamo- 
genesis in  animals,  is  still  more  difficult  to  disentangle.  Per- 
haps the  evidence  most  to  the  point  is  furnished  by  the  contrast 
between  Man  and  certain  other  Mammals  approaching  to  him 
in  mass.  To  compare  him  with  the  domestic  Sheep,  which, 
though  not  very  unlike  in  size,  is  relatively  prolific,  is  objec- 
jectionable  because  of  the  relative  inactivity  of  Sheep ;  and 
this,  too,  may  be  alleged  as  a  reason  why  the  Ox,  though  fur 
more  bulky,  is  also  far  more  fertile,  than  Man.  Further, 
against  a  comparison  with  the  Horse,  which,  while  both  larger 
and  more  prolific,  is  tolerably  active,  it  may  be  urged  that,  in 
his  case,  and  the  cases  of  herbivorous  creatures  generally,  the 
small  exertion  required  to  procure  food,  joined  with  the  great 
ratio  borne  by  the  assimilative  organs  to  the  organs  they  have 
to  build  up  and  repair,  vitiates  the  result.  "We  may,  however, 
fairly  draw  a  parallel  between  Man  and  a  large  carnivore.  The 
Lion,  superior  in  size,  and  perhaps  equal  in  activity,  has  a 
digestive  system  not  proportionately  greater;  and  yet 'has  a 
higher  rate  of  multiplication  than  Man.  Here  the  only  de- 
cided want  of  parity,  besides  that  of  organization,  is  that  of 
food.  Possibly  a  carnivore  gains  an  advantage  in  having  a 


DEVELCPMEKT   AND    GENESIS.  445 

surplus  nutriment  consisting  almost  wholly  of  those  nitro- 
genous materials  from  which  the  bodies  of  young  ones  are 
mainly  formed.  But,  allowing  for  all  other  differences,  it 
appears  not  improbable  that  the  smallness  of  human  fertility 
compared  with  the  fertility  of  large  feline  animals,  is  due  to 
the  greater  complexity  of  the  human  organization — more 
especially  the  organization  of  the  nervous  system.  Taking 
degree  of  nervous  organization  as  the  chief  correlative  of 
mental  capacity  ;  and  remembering  the  physiological  cost  of 
that  discipline  whereby  high  mental  capacity  is  reached ;  we 
may  suspect  that  nervous  organization  is  very  expensive  :  the 
inference  being  that  bringing  it  up  to  the  level  it  reaches  in 
Man,  whose  digestive  system,  by  no  means  large,  has  at  the 
same  time  to  supply  materials  for  general  growth  and  daily 
waste,  involves  a  great  retardation  of  maturity  and  sexual 
genesis. 


CHAPTER  VIII. 

ANTAGONISM   BETWEEN   EXPENDITURE   AND    GENESIS. 

§  347.  Under  this  head  we  have  to  set  down  no  evidence 
derived  from  the  vegetal  kingdom.  Plants  are  not  expenders 
of  force  in  such  degrees  as  to  affect  the  general  relations  with 
which  we  are  dealing.  They  have  not  to  maintain  a  heat 
above  that  of  their  environment ;  nor  have  they  to  generate 
motion ;  and  hence  consumption  for  these  two  purposes  does 
not  diminish  the  stock  of  material  that  serves  on  the  one 
hand  for  growth  and  on  the  other  hand  for  propagation. 

It  will  be  well,  too,  if  we  pass  over  the  lower  animals : 
especially  those  aquatic  ones  which,  being  nearly  of  the 
same  temperature  as  the  water,  and  nearly  of  the  same 
specific  gravity,  lose  but  little  in  evolving  motion,  sensible 
and  insensible.  A  further  reason  for  excluding  from  con- 
sideration these  inferior  types,  is,  that  we  do  not  know  enough 
of  their  rates  of  genesis  to  permit  of  our  making,  with  any 
satisfaction,  those  involved  comparisons  here  to  be  entered 
upon. 

The  facts  on  which  we  must  mainly  depend  are  those  to  be 
gathered  from  terrestrial  animals ;  and  chiefly  from  those 
higher  classes  of  them  which  are  at  the  same  time  great 
expenders  and  have  rates  of  multiplication  about  which  our 
knowledge  is  tolerably  definite.  "We  will  restrict  ourselves, 
then,  to  the  evidence  which  Birds  and  Mammals  supply 

§  348.  Satisfactory  proof   that  loss   of  substance   in   the 


EXPENDITURE    AND    GENESIS.  447 

maintenance  of  heat  diminishes  the  rapidity  of  propagation, 
is  difficult  to  obtain.  It  is,  indeed,  obvious  that  the  warm- 
blooded Vertelrata  are  less  prolific  than  the  cold-blooded ; 
but  then  they  are  at  the  same  time  more  vivacious.  Similarly, 
between  Mammals  and  Birds  (which  are  the  warmer- blooded 
of  the  two)  there  is,  other  things  equal,  a  parallel,  though 
much  smaller,  difference ;  but  here,  too,  the  unlikenesses  of 
muscular  action  complicate  the  evidence.  Again,  the  annual 
return  of  generative  activity  has  an  average  correspondence 
with  the  annual  return  of  a  warmer  season,  which,  did  it 
stand  alone,  might  be  taken  as  evidence  that  a  diminished 
cost  of  heat-maintenance  leads  to  such  a  surplus  as  makes 
reproduction  possible.  But  then,  this  periodic  rise  of  tem- 
perature is  habitually  accompanied  by  an  increase  in  the 
quantity  of  food — a  factor  of  equal  or  greater  importance. 
We  must  be  content,  therefore,  with  such  few  special  facts 
as  admit  of  being  disentangled. 

Certain  of  these  we  are  introduced  to  by  the  general  rela- 
tion last  named — the  habitual  recurrence  of  genesis  with  the 
recurrence  of  spring.  For  in  some  cases  a  domesticated  crea- 
ture has  its  supplies  of  food  almost  equalized ;  and  hence  the 
effect  of  varying  nutrition  may  be  in  great  part  eliminated 
from  the  comparison.  The  common  Fowl  yields  an  illustra- 
tion. It  is  fed  through  the  cold  months,  but  nevertheless,  in 
mid- winter,  it  either  wholly  leaves  off  laying  or  lays  very 
sparingly.  And  then  we  have  the  further  evidence  that  if  it 
lays  sparingly,  it  does  so  only  on  condition  that  the  heat,  as 
well  as  the  food,  is  artificially  maintained.  Hens  lay  in  cold 
weather  only  when  they  are  kept  warm.  To  which  fact  may 
be  added  the  kindred  one  that  "  when  pigeons  receive  arti- 
ficial heat,  they  not  only  continue  to  hatch  longer  in  autumn, 
"but  will  recommence  in  spring  sooner  than  they  would  other- 
wise do."  An  analogous  piece  of  evidence  is  that,  in 
winter,  inadequately-sheltered  Cows  either  cease  to  give  milk 
or  give  it  in  diminished  quantity.  For  though  giving  milk 
is  not  the  same  thing  as  bearing  a  young  one,  yet,  as  milk 


Ii8  LAWS    OF    MULTll'LlCATIOX. 

is  part  of  the  material  from  which  a  }roung  one  5s  built  up, 
it  is  part  of  the  outlay  for  reproductive  purposes,  and  diminu- 
tion of  it  is  a  loss  of  reproductive  power.  Indeed  the  case 
aptly  illustrates,  under  another  aspect,  the  struggle  between 
self-preservation  and  race-preservation.  Maintenance  of  ths 
cow's  life  depends  on  maintenance  of  its  heat ;  and  main- 
tenance of  its  heat  may  entail  such  reduction  in  the  supply 
of  milk  as  to  cause  the  death  of  the  calf. 

Evidence  derived  from  the  habits  of  the  same  or  allied 
genera  in  different  climates,  may  naturally  be  looked  for ;  but 
it  is  difficult  to  get,  and  it  can  scarcely  be  expected  that  the 
remaining  conditions  of  existence  will  be  so  far  similar  as  to 
allow  of  a  fair  comparison  being  made.  The  only  illustrative 
facts  I  have  met  with  which  seem  noteworthy,  are  some  named 
by  Mr.  Gould  in  his  work  on  The  Birds  of  Australia.  lie 
says : — "  I  must  not  omit  to  mention,  too,  the  extraordinary 
fecundity  which  prevails  in  Australia,  many  of  its  smaller 
birds  breeding  three  or  four  times  in  a  season ;  but  laying 
fewer  eggs  in  the  early  spring  when  insect  life  is  less 
developed,  and  a  greater  number  later  in  the  season,  when 
the  supply  of  insect  food  has  become  more  abundant.  I  have 
also  some  reason  to  believe  that  the  young  of  many  species 
breed  during  the  first  season,  for  among  others,  I  frequently 
found  one  section  of  the  Honey-eaters  (the  Melithrcpti) 
sitting  upon  eggs  while  still  clothed  in  the  brown  dress  of 
immaturity  ;  and  we  know  that  such  is  the  case  with  the 
introduced  Gbllinaccce  (or  poultry)  three  or  four  generations 
of  which  have  been  often  produced  in  the  course  of  a  year.  " 
Though  here  Mr.  Gould  refers  only  to  variation  in  the 
quantity  of  food  as  a  cause  of  variation  in  the  rate  of 
multiplication,  may  we  not  suspect  that  the  warmth  is  r. 
part-cause  of  the  high  rate  which  he  describes  as  general? 

§  349.  Of  the  inverse  variation  between  activity  and 
genesis,  we  get  clear  proof.  Let  us  begin  with  that  which 
Birds  furnish. 


EXPENDITURE    AND    GENESIS.  449 

First  we  have  the  average  contrast,  already  hinted,  between 
the  fertility  of  Birds  and  the  fertility  of  Mammals.  Compar- 
ing the  large  with  the  large  and  the  small  with  the  small,  we 
see  that  creatures  which  continually  go  through  the  muscular 
exertion  of  sustaining  themselves  in  the  air  and  propelling 
themselves  rapidly  through  it,  are  less  prolific  than  creatures 
of  equal  weights  which  go  though  the  smaller  exertion  of 
moving  about  over  solid  surfaces.  Predatory  Birds  have 
fewer  young  ones  than  predatory  Mammals  of  approximately 
the  same  sizes.  If  we  compare  Rooks  with  Rats,  or  Finches 
with  Mice,  we  find  like  differences.  And  these  differences  are 
greater  than  at  first  appears.  For  whereas  among  Mammals 
a  mother  is  able,  unaided,  to  bear  and  suckle  and  rear  half- 
way to  maturity,  a  brood  that  probably  weighs  more  in  pro- 
portion than  does  the  brood  of  a  Bird  ;  a  Bird,  or  at  least  a 
Bird  that  flies  much,  is  unable  to  do  this.  Both  parents  have 
to  help ;  and  this  indicates  that  the  margin  for  reproduction 
in  each  adult  individual  is  smaller. 

Among  Birds  themselves  occur  contrasts  which  may  be 
next  considered.  In  the  Raptorial  class,  various  species  of 
which,  differing  in  their  sizes,  are  similarly  active  in  their 
habits,  we  see  that  the  small  are  more  prolific  than  the  large. 
The  Golden  Eagle  has  usually  2  eggs :  sometimes  only  1. 
As  we  descend  to  the  Kites  and  Falcons,  the  number  is  2  or 
or  3,  and  3  or  4.  And  when  we  come  to  the  Sparrow-Hawk, 
3  to  5  is  the  specified  number.  Similarly  among  the  Owls  : 
while  the  Great  Eagle-Owl  has  2  or  3  eggs,  the  comparatively 
small  Common  Owl  has  4  or  5.  As  before  hinted,  it  is  im- 
possible to  say  what  proportions  of  these  differences  are  due 
to  unlikenesses  of  bulk  merely,  and  what  proportions  are  due 
to  unlikenesses  in  the  costs  of  locomotion.  But  we  may  fairly 
assume  that  the  unlikenesses  in  the  costs  of  locomotion  are 
here  the  more  important  factors.  Weights  varying  as  the 
cubes  of  the  dimensions,  while  muscular  powers  vary  as  the 
squares,  the  expense  of  flight  increases  more  rapidly  than  the 
size  increases ;  and  as  motion  through  the  air  requires  more 


4:50  LAWS   OF    MULTIPLICATION. 

effort  than  motion  on  the  ground,  this  geometric  tl  progression 
tells  more  rapidly  on  Birds  than  on  Mammals.  Be  this  as  it 
may,  however,  these  contrasts  support  the  argument ;  as  do 
various  others  that  may  be  set  down.  The  Finch  family,  for 
example,  have  broods  averaging  about  5  in  number,  and  have 
commonly  2  broods  in  the  season  ;  while  in  the  Crow  family 
the  number  of  the  brood  is  on  the  average  less,  and  there  is 
but  one  brood  in  a  season.  And  then  on  descending  to  such 
small  birds  as  the  Wrens  and  the  Tits,  we  have  8,  10,  12  to 
]  5  eggs,  and  often  two  broods  in  the  year.  One  of  the  best 
illustrations  is  furnished  by  the  Swallow-tribe,  throughout 
which  there  is  little  or  no  difference  in  mode  of  life  or  in  food. 
The  Sand-Martin,  much  the  least  of  them,  has  usually  G  eggs  ; 
the  Swallow,  somewhat  larger,  has  4  or  5 ;  and  the  Swift, 
larger  still,  has  but  2.  Here  we  see  a  lower  fertility  associated 
in  part  with  greater  size,  but  associated  still  more  con- 
spicuously with  greater  expenditure.  For  the  difference  of 
fertility  is  more  than  proportionate  to  the  difference  of  bulk, 
as  shown  in  other  cases  ;  and  for  this  greater  difference  there 
is  the  reason,  that  the  Swift  has  to  support  not  only  the  cost 
of  propelling  its  larger  mass  through  the  air,  but  also  the  cost 
of  propelling  it  at  a  higher  velocity. 

Omitting  much  evidence  of  like  nature,  let  us  note  that 
disclosed  by  comparisons  of  certain  groups  of  birds  with  other 
groups.  "Skulkers"  is  the  descriptive  title  applied  to  the 
Water-Rail,  the  Corn- Crake,  and  their  allies,  which  evade 
enemies  by  concealment — consequently  expending  but  little 
in  locomotion.  These  birds  have  relatively  large  broods — 6 
to  11,  8  to  12,  &c.  Not  less  instructive  are  the  contrasts  be- 
tween the  Gallinaceous  Birds  and  other  Birds  of  like  sizes  but 
more  active  habits.  The  Partridge  and  the  Wood-Pigeon  are 
about  equal  in  bulk,  and  have  much  the  same  food.  Yet  while 
the  one  has  from  10  to  15  young  ones,  the  other  has  but  2 
young  ones  twice  a-year :  its  annual  reproduction  is  but 
one-third.  It  may  be  said  that  the  ability  of  the  Partridge 
to  bring  up  so  large  a  brood,  is  due  to  that  habit  of  its  tribe 


EXPENDITURE   AND   GENESIS.  451 

which  one  of  its  names,  "  Scrapers,"  describes ;  and  to  the 
accompanying  habit  of  the  young,  which  begin  to  get  their 
own  living  as  soon  as  they  are  hatched :  so  saving  the  parents' 
labour.  Conversely,  it  may  be  sail  that  the  inability  of  the 
Pigeon  to  rear  more  than  2  at  a  time,  is  caused  by  the  necessity 
of  fetching  everything  they  eat.  But  the  alleged  relation 
holds  nevertheless.  On  the  one  hand,  a  great  part  of  the  food 
which  the  Partridge  chicks  pick  ap,  is  food  which,  in  their 
absence,  the  mother  would  have  picked  up :  though  each  chick 
costs  her  far  less  than  a  young  Pigeon  costs  its  parents,  yet 
the  whole  of  her  chicks  cost  her  a  great  deal  in  the  shape  of 
abstinence — an  abstinence  she  can  bear  because  she  has  to  fly 
but  little.  On  the  other  hand,  the  Pigeon's  habit  of  laying 
and  hatching  but  two  eggs,  must  not  be  referred  to  any  fore- 
seen necessity  of  going  through  so  much  labour  in  supporting 
the  young,  but  to  a  constitutional  tendency  established  by  such 
labour.  This  is  proved  by  the  curious  fact  that  when  do- 
mesticated, and  saved  from  such  labour  by  artificial  feeding, 
Pigeons,  says  Macgillivray,  "  are  frequently  seen  sitting  on 
eggs  long  before  the  former  brood  is  able  to  leave  the  nest,  so 
that  the  parent  bird  has  at  the  same  time  young  birds  and 
eggs  to  take  care  of." 

§  350.  Made  to  illustrate  the  effect  of  activity  on  fertility, 
most  comparisons  among  Mammals  are  objectionable:  other  cir- 
cumstances are  not  equal.  A  few,  however,  escape  this  criticism. 

One  is  that  between  the  Hare  and  the  Rabbit.  These  are 
closely- allied  species  of  the  same  genus,  similar  in  their  diet 
but  unlike  in  their  expenditures  for  locomotion.  The  rela- 
tively-inert llabbit  has  5  to  8  young  ones  in  a  litter,  and 
several  litters  a-year ;  "while  the  relatively- active  Hare  has 
but  2  to  5  in  a  litter.  This  is  not  all.  The  Ptabbit  begins 
to  breed  at  six  months  old;  but  a  year  elapses  before  the 
Hare  begins  to  breed.  These  two  factors  compounded,  result 
in  a  difference  of  fertility  far  greater  than  can  be  ascribed  to 
unlikcncss  of  the  two  creatures  in  size. 


i52  LAWS   OF    MULTIPLICATION. 

Perhaps  the  most  striking  piece  of  evidence  which  Mam- 
mals furnish,  is  the  extreme  infertility  of  our  common  Bat. 
The  Cheiroptera  and  the  Rodentia  are  very  similar  in  their 
internal  structures.  Diversity  of  constitution,  therefore, 
cannot  vitiate  the  comparison  between  Bats  and  Mice,  which 
are  about  the  same  in  size.  Though  their  diets  differ,  the 
difference  is  in  favour  of  the  Bat :  its  food  being  exclusively 
animal  while  that  of  the  Mouse  is  mainly  vegetal.  What 
now  are  their  respective  rates  of  genesis  ?  The  Mouse  pro- 
duces many  young  at  a  time,  reaching  even  10  or  12  ;  while 
the  Bat  produces  only  one  at  a  time.  Whether  the  Bat 
repeats  its  one  more  frequently  than  the  Mouse  repeats  its 
ten  is  not  stated ;  but  it  is  quite  certain  that  even  if  it  does 
so,  the  more  frequent  repetition  cannot  be  such  as  to  raise  its 
fertility  to  anything  like  that  of  the  Mouse.  And  this 
relatively- low  rate  of  multiplication  we  may  fairly  ascribe  to 
its  relatively- high  rate  of  expenditure. 

Here  let  us  note,  in  passing,  an  interesting  example  of  the 
way  in  which  a  species  that  has  no  specially- great  power  of 
self-preservation,  while  its  power  of  multiplication  is  extremely 
small,  nevertheless  avoids  extinction  because  it  has  to  meet 
an  unusually-small  total  of  race- destroy  ing  forces.  Leaving 
out  parasites,  the  only  enemy  of  the  Bat  is  the  Owl ;  and  tho 
Owl  is  sparingly  distributed. 

§  351.  These  general  evidences  may  be  enforced  by  some 
special  evidences.  We  have  few  opportunities  of  observing 
how,  within  the  same  species,  variations  of  expenditure  are 
related  to  variations  of  fertility.  But  a  fact  or  two  showing 
the  connexion  may  be  named. 

Doctor  Duncan  quotes  a  statement. to  the  point  respecting 
the  breeding  of  dogs.  Already  in  §  341  I  have  extracted  a  part 
of  this  statement,  to  the  effect  that  before  her  growth  is  com- 
plete, a  bitch  bears  at  a  birth  fewer  puppies  than  when  she 
becomes  full-grown.  An  accompanying  allegatio"n  is,  that 
her  declining  vigour  is  shown  by  a  decrease  in  the  number  of 


EXPENDITURE   AND   GENESIS.  453 

puppies  contained  in  a  litter,  "  ending  in  one  or  two."  And 
then  it  is  further  alleged  that,  "  as  regards  the  amount  of 
work  a  dog  has  to  perform,  so  will  the  decline  be  rapid  or 
gradual ;  and  hence,  if  a  bitch  is  worked  hard  year  after  year, 
she  will  fail  rapidly,  and  the  diminution  of  her  puppies  will 
be  accordingly;  but  if  worked  moderately  and  well  kept,  she 
will  fail  gradually,  and  the  diminution  will  be  less  rapid." 

In  this  place,  more  fitly  than  elsewhere,  may  be  added  a 
fact  of  like  implication,  though  of  a  different  order.  Of  course 
whether  excessive  expenditure  be  in  the  continual  repairs  of 
nervo-muscular  tissues  or  in  replacing  other  tissues,  the  re- 
active effects,  if  not  quite  the  same,  will  be  similar — there 
will  be  a  decrease  of  the  surplus  available  for  genesis.  If, 
then,  in  any  animals  there  from  time  to  time  occur  unusual 
outlays  for  self-maintenance,  we  may  expect  the  periods  of 
such  outlays  to  be  periods  of  diminished  or  arrested  repro- 
duction. That  they  are  so  the  moulting  of  birds  shows  us. 
When  hens  begin  to  moult  they  cease  to  lay.  While  they 
are  expending  so  much  in  producing  new  clothing,  they  have 
nothing  to  expend  for  producing  eggs. 


CHAPTER  IX. 

COINCIDENCE  BETWEEN  HIGH  NUTRITION  AND  GENESIS. 

§  352.  Under  this  head  may  be  grouped  various  facts 
which,  in  another  way,  tell  the  same  tale  as  those  contained 
in  the  last  chapter.  The  evidence  there  put  together  went  to 
show  that  increased  cost  of  self-maintenance  entailed  de- 
creased power  of  propagation.  The  evidence  to  be  set  down 
here,  will  go  to  show  that  power  of  propagation  is  augmented 
by  making  self- maintenance  unusually  easy.  For  into  this 
may  be  translated  the  effect  of  abundant  food. 

To  put  the  proposition  more  specifically — we  have  seen 
that  after  individual  growth,  development,  and  daily  con- 
sumption have  been  provided  for,  the  surplus  nutriment 
measures  the  rate  of  multiplication.  This  surplus  may  be 
raised  in  amount  by  such  changes  in  the  environment  as 
bring  a  larger  supply  of  the  materials  or  forces  on  which 
both  parental  life  and  the  lives  of  offspring  depend.  Be 
there,  or  be  there  not,  any  expenditure,  a  higher  nutrition 
will  make  possible  a  greater  propagation.  We  may  expect 
this  to  hold  both  of  agamogenesis  and  of  gamogenesis ;  and 
we  shall  find  that  it  does  so. 

§  353.  On  multi-axial  plants,  the  primary  effect  of  surplus 
nutriment  is  a  production  of  large  and  numerous  leaf-shoots. 
How  this  asexual  multiplication  results  from  excessive  nutri- 
tion, is  well  shown  when  the  leading  axis,  or  a  chief  branch,  is 
broken  off  towards  its  extremity.  The  axillary  buds  below 


NUTRITION    AND   GENESIS.  455 

the  breakage  quickly  swell  and  burst  into  lateral  shoots, 
which  often  put  forth  secondary  shoots  :  two  generations  of 
agamic  individuals  arise  where  there  probably  would  have 
been  none  but  for  the  local  abundance  of  sap,  no  longer 
drawn  off.  In  like  manner  the  abnormal  agamogenesis  which 
we  have  in  proliferous  flowers,  is  habitually  accompanied  by 
a  general  luxuriance,  implying  an  unusual  plethora. 

No  less  conclusive  is  the  evidence  furnished  by  agamo- 
genesis in  animals.  Sir  John  Dalyell,  speaking  of  Hydra 
tula,  whose  peculiar  metagenesis  he  was  the  first  to  point  out, 
says — "  It  is  singular  how  much  propagation  is  promoted  by 
abundant  sustenance."  This  Polype  goes  on  budding- out  young 
polypes  from  its  sides,  with  a  rapidity  proportionate  to  the 
supply  of  materials.  So,  too,  is  it  with  the  agamic 

reproduction  of  the  Aphis.  As  cited  by  Professor  Huxley, 
Kyber  "  states  that  he  raised  viviparous  broods  of  both  this 
species  (Aphis  Dianthi}  and  A.  Hosce  for  four  consecutive 
years,  without  any  intervention  of  males  or  oviparous  females, 
and  that  the  energy  of  the  power  of  agamic  reproduction  was 
at  the  end  of  that  period  undiminished.  The  rapidity  of  the 
agamic  prolification  throughout  the  whole  period  was  directly 
proportional  to  the  amount  of  warmth  and  food  supplied." 

In  these  cases  the  relation  is  not  appreciably  complicated  by 
expenditure.  The  parent  having  reached  its  limit  of  growth, 
the  absorbed  food  goes  to  asexual  multiplication:  scarcely 
any  being  deducted  for  the  maintenance  of  parental  life. 

§  354.  The  sexual  multiplication  of  organisms  under 
changed  conditions,  undergoes  variations  conforming  to  a 
parallel  law.  Cultivated  plants  and  domesticated  animals 
yield  us  proof  of  this. 

Facts  showing  that  in  cultivated  plants,  sexual  genesis  in- 
creases with  nutrition,  are  obscured  by  facts  showing  that  a 
less  rapid  asexual  genesis,  and  an  incipient  sexual  genesis,  ac- 
company the  fall  from  a  high  to  a  moderate  nutrition.  The 
confounding  of  these  two  relations  has  led  to  mistaken  infer- 
VOL.  II.  20 


456  LAWS    OF    MULTIPLICATION. 

ences.  When  treating  of  Genesis  inductively,  we  reached  the 
generalization  that  "  the  products  of  a  fertilized  germ  go 
on  accumulating  by  simple  growth,  so  long  as  the  forces 
whence  growth  results  are  greatly  in  excess  of  the  antagonist 
forces ;  but  that  when  diminution  of  the  one  set  of  forces,  or 
increase  of  the  other,  causes  a  considerable  decline  in  this  ex- 
cess, and  an  approach  towards  equilibrium,  fertilized  germs 
are  again  produced."  (§  78.)  It  was  pointed  out  that  this 
holds  of  organisms  which  multiply  by  heterogenesis,  as 
well  as  those  which  multiply  by  homogenesis.  And  plants 
were  referred  to  as  illustrating,  both  generally  and  locally, 
the  decline  of  agamic  multiplication  and  commencement  of 
gamic  multiplication,  along  with  a  lessening  rate  of  nutrition. 
Now  the  many  cases  that  are  given  of  fruitfulness  caused  in 
trees  by  depletion,  are  really  cases  of  this  change  from 
agamogenesis  to  gamogenesis ;  and  simply  go  to  prove  that 
what  would  naturally  arise  when  decreased  peripheral  growth 
had  followed  increased  size,  may  be  brought  about  artificially 
by  diminishing  the  supply  of  materials  for  growth.  Cramp- 
ing its  roots  in  a  pot,  or  cutting  them,  or  ringing  its  branches, 
will  make  a  tree  bear  very  early :  bringing  about  a  pre- 
mature establishment  of  that  relative  innutrition  which 
would  have  spontaneously  arisen  in  course  of  time.  Such 
facts  by  no  means  show  that  in  plants,  sexual  genesis  in- 
creases as  nutrition  diminishes.  When  it  has  once  set  in, 
sexual  genesis  is  scanty  or  imperfect  unless  nutrition  is  good. 
Though  the  starved  plant  mav  blossom,  yet  many  of  its 
blossoms  will  fail ;  and  such  seeds  as  it  produces  will  be  ill- 
furnished  with  those  enveloping  structures  and  that  store  of 
albumen,  &c.,  needed  to  give  good  chances  of  successful  germi- 
nation— the  number  of  surviving  offspring  will  be  diminished. 
Were  it  otherwise,  the  manuring  of  fields  that  are  to  bear 
seed-crops,  would  be  not  simply  useless  but  injurious.  Were 
it  otherwise,  dunging  the  roots  of  a  fruit-tree  would  in  all 
cases  be  impolitic ;  instead  of  being  impolitic  only  where  the 
growth  of  sexless  axes  is  still  luxuriant.  Were  it  otherwise, 


NUTRITION    AND    GENESIS.  457 

a  tree  which  has  borne  a  heavy  crop,  should,  by  the  con- 
sequent depletion,  be  led  to  bear  a  still  heavier  crop  next 
year ;  whereas  it  is  apt  to  be  wholly  or  partially  barren  next 
year — has  to  recover  a  state  of  tolerably-high  nutrition 
before  its  sexual  genesis  again  becomes  large. 

But  the  best  illustrations  are  those  yielded  by  animals, 
in  which  we  have,  besides  an  increased  supply  of  nutriment, 
a  diminished  expenditure.  Two  classes  of  comparisons,  alike 
in  their  implications,  may  be  made — comparisons  between 
tame  and  wild  animals  of  the  same  species  or  genus,  and  com- 
parisons between  tame  animals  of  the  same  species  differently 
treated. 

To  begin  with  Birds,  let  us  first  contrast  the  farm-yard 
GallinacecB  with  their  kindred  of  the  fields  and  woods.  Not- 
withstanding their  greater  size,  which,  other  things  equal, 
should  be  accompanied  by  smaller  fertility,  the  domesticated 
kinds  have  more  numerous  offspring  than  the  wild  kinds.  A 
Turkey  has  a  dozen  in  a  brood,  while  a  Pheasant  has  from  6 
to  10.  Twice  or  thrice  in  a  season,  a  Hen  rears  as  many 
chickens  as  a  Partridge  rears  once  in  a  season.  Anserine  birds 
show  us  parallel  differences.  The  Tame  Goose  sits  on  12  or 
more  eggs,  but  the  Wild  Goose  sits  on  5,  6,  or  7  ;  and  theae 
are  noted  as  considerably  smaller.  It  is  the  same  with  Ducks : 
the  domesticated  variety  lays  and  hatches  twice  as  many  eggs 
as  the  wild  variety.  And  the  like  holds  of  Pigeons.  After 
remarking  of  the  Cohnnba  licia  that  "  in  spring  when  they 
have  plenty  of  corn  to  pick  from  the  newly-sown  fields,  they 
begin  to  get  fat  and  pair ;  and  again,  in  harvest,  when  the 
corn  is  cut  down, "  Macgillivray  goes  on  to  say,  that  "  the 
same  pair  when  tamed  generally  breed  four  times"  in  the 
year.  That  between  different  poultry-yards,  in- 

equalities of  fertility  are  caused  by  inequalities  in  the  supplies 
of  food,  is  a  familiar  truth.  High  feeding  shows  its  effects  not 
only  in  the  continuous  laying,  but  also  in  the  sizes  of  the 
e^gs.  Among  directions  given  for  obtaining  eggs  from 
pullets  late  in  the  year,  it  is  especially  insisted  on  that  they 


158  LAWS   OF   MULTIPLICATION. 

shall  have  a  generous  diet.  Respecting  Pigeons  Macgillivray 
writes : — "  that  their  breeding  depends  much  on  their 
having  plenty  of  food  to  fatten  them,  seems,  I  think, 
evident  from  the  circumstance  that,  when  tamed,  which 
they  easily  are,  they  are  observed  to  breed  in  every  month  of 
the  year.  I  do  not  mean  that  the  same  pair  will  breed  every 
month;  but  some  in  the  flock,  if  well  fed,  will  breed  at  any 
season."  There  may  be  added  a  fact  of  like  meaning 

which  partially-domesticated  birds  yield.  The  Sparrow  is  one 
of  the  Finch  tribe  that  has  taken  to  the  neighbourhood  of 
houses ;  and  by  its  boldness  secures  food  not  available  to  its 
congeners.  The  result  is  that  it  has  several  broods  in  a  sea- 
son, while  its  field-haunting  kindred  have  none  of  them  more 
than  two  broods,  and  some  have  only  one. 

Equally  clear  proof  that  abundant  nutriment  raises  the  rate 
of  multiplication,  occurs  among  Mammals.  Compare  the 
litters  of  the  Dog  with  the  litters  of  the  Wolf  and  the  Fox. 
Whereas  those  of  the  one  range  in  number  from  6  to  14,  the 
others  contain  respectively  5  or  6  or  occasionally  7,  and 
4  or  5  or  rarely  6.  Again,  the  wild  Cat  has  4  or  5  kittens  ; 
but  the  tame  Cat  has  5  or  6  kittens  2  or  3  times  a-year. 
So,  too,  is  it  with  the  Weasel  tribe.  The  Stoat  has  5 
young  ones  once  a-year.  The  Ferret  has  2  litters  yearly, 
each  containing  from  6  to  9  ;  and  this  notwithstanding  that 
it  is  the  larger  of  the  two.  Perhaps  the  most  striking 
contrast  is  that  between  the  wild  and  tame  varieties  of 
the  Pig.  While  the  one  produces,  according  to  its  age,  from 
4  to  8  or  10  young  ones,  once  a  year,  the  other  produces 
sometimes  as  many  as  17  in  a  litter ;  or,  in  other  cases,  will 
bring  up  5  litters  of  10  each  in  two  years — a  rate  of  reproduc- 
tion that  is  unparalleled  in  animals  of  as  large  a  size. 
And  let  us  not  omit  to  note  that  this  excessive  fertility 
occurs  where  there  is  the  greatest  inactivity — where  there  is 
plenty  to  eat  and  nothing  to  do.  There  is 

no  less  distinct  evidence  that  among  domesticated  Mammals 
themselves,  the  well-fed  individuals  are  more  prolific  than 


NUTRITION   AND   GENESIS.  459 

the  ill-fed  individuals.  On  the  high  and  comparatively- 
infertile  Cotswolds,  it  is  unusual  for  E\ves  to  have  twius;  but 
they  very  commonly  have  twins  in  the  adjacent  rich  valley  of 
the  Severn.  Similarly,  among  the  barren  hills  of  the  west  of 
Scotland,  two  lambs  will  be  borne  by  about  one  Ewe  in  twenty  ; 
whereas  in  England,  something  like  one  Ewe  in  three  will 
bear  two  lambs.  Nay,  in  rich  pastures,  twins  are  more 
frequent  than  single  births;  and  it  occasionally  happens 
that,  after  a  genial  autumn  and  consequent  good  grazing,  a 
flock  of  Ewes  will  next  spring  yield  double  their  number  of 
iambs — the  triplets  balancing  the  uniparae.  So  direct  is  this 
relation,  that  I  have  heard  a  farmer  assert  his  ability  to  fore- 
tell, from  the  high,  medium,  or  low,  condition  of  an  Ewe  in 
the  autumn,  whether  she  will  next  spring  bear  two,  or  one, 
or  none. 

§  355.  An  objection  must  here  be  met.  Many  facts  may 
be  brought  to  prove  that  fatness  is  not  accompanied  by  ferti- 
lity but  by  barrenness  ;  and  the  inference  drawn  is  that  high 
feeding  is  unfavourable  to  genesis.  The  premiss  may  be 
admitted  while  the  conclusion  is  denied. 

There  is  a  distinction  between  what  may  be  called  normal 
plethora,  and  an  abnormal  plethora,  liable  to  be  confounded 
with  it.  The  one  is  a  mark  of  constitutional  wealth ;  but  the 
other  is  a  mark  of  constitutional  poverty.  Normal  plethora 
is  a  superfluity  of  materials  both  for  the  building  up  of 
tissue  and  the  evolution  of  force ;  and  this  is  the  plethora 
which  we  have  found  to  be  associated  with  unusual  fecundity. 
Abnormal  plethora,  which,  as  truly  alleged,  is  accompanied 
by  infecundity,  is  a  superfluity  of  force- evolving  materials 
joined  with  either  a  positive  or  a  relative  deficiency  of  tissue- 
forming  materials  :  the  increased  bulk  indicating  this  state, 
being  really  the  bulk  of  so  much  inert  or  dead  matter.  Note, 
first,  a  few  of  the  facts  which  show  us  that  obesity  implies 
physiological  impoverishment. 

Neither  in  brutes  nor  men  does  it  ordinarily  occur  either 


i60  LAWS   OF    MULTIPLICATION. 

in  youth  or  in  that  early  maturity  during  which  the  vigour 
is  the  greatest  and  the  digestion  the  best :  it  does  not 
habitually  accompany  the  highest  power  of  taking  up  nutri- 
tive materials.  When  fatness  arises  in  the  prime  of  life, 
whether  from  peculiarity  of  food  or  other  circumstance,  it  is 
not  the  sign  of  an  increased  total  vitality.  On  the  contrary,  if 
great  muscular  action  has  to  be  gone  through,  the  fat  must 
be  got  rid  of — either,  as  in  a  man,  by  training,  or  as  in  a 
horse  that  has  grown  bulky  while  out  at  grass,  by  putting 
him  on  such  more  nutritive  diet  as  oats.  The 

frequency  of  senile  fatness,  both  in  domesticated  creatures 
and  in  ourselves,  has  a  similar  implication.  Whether  we 
consider  the  smaller  ability  of  those  who  display  it  to  with- 
stand large  demands  on  their  powers,  or  whether  we  consider 
the  comparatively-inferior  digestion  common  among  them, 
we  see  that  the  increased  size  indicates,  not  an  abundance  of 
materials  which  the  organ  ism.  requires,  but  an  abundance  of 
materials  which  it  does  not  require.  Of  like  mean- 

ing is  the  fact  that  women  who  have  had  several  children, 
and  animals  after  they  have  gone  on  bearing  young  for  some 
time,  frequently  become  fut;  and  lose  their  fecundity  as 
they  do  this.  In  such  cases,  the  fatness  is  not  to  be  taken  as 
the  cause  of  the  infecundity ;  but  the  constitutional  ex- 
haustion which  the  previous  production  of  offspring  has  left, 
Bhows  itself  at  once  in  the  failing  fecundity  and  the  com- 
mencing fatness.  There  is  yet  another  kind  of  evidence. 
Obesity  not  uncommonly  sets  in  after  the  system  has 
been  subject  to  debilitating  influences.  Often  a  serious  illness 
is  followed  by  a  corpulence  to  which  there  was  previously  no 
tendency.  And  the  prolonged  administration  of  mercury,  con- 
stitutionally injurious  as  it  is,  sometimes  produces  a  like  effect. 
Closer  inquiry  verifies  the  conclusion  to  which  these  facts 
point.  The  microscope  shows  that  along  with  the  increase  of 
bulk  common  in  advanced  life,  there  goes  on  what  is  called 
"  fatty  degeneration :"  oil-globules  are  deposited  where  there 
should  be  particles  of  flesh — or  rather,  we  may  say,  the  hydro- 


NUTRITION    AND    GENESIS.  461 

carbonaceous  molecules  locally  produced  by  decomposition  of 
the  nitrogenous  molecules,  have  not  been  replaced  by  other 
nitrogenous  molecules,  as  they  should  have  been.  This  fatty 
degeneration  is,  indeed,  a  kind  of  local  death.  For  so  regard- 
ing it  we  have  not  simply  the  reason  that  an  active  substance 
has  its  place  occupied  by  an  inert  substance ;  but  we  have 
the  reason  that  the  flesh  of  dead  bodies,  under  certain 
conditions,  is  transformed  into  a  fatty  matter  called  adipocere. 
The  infertility  that  accompanies  fatness  in  domestic  animals, 
has,  however,  othsr  causes  than  that  declining  constitutional 
vigour  which  the  fatness  indicates.  Being  artificially  fed,  these 
animals  cannot  always  obtain  what  their  systems  need.  That 
which  is  given  to  them  is  often  given  expressly  because  of  its 
fattening  quality.  And  since  the  capacity  of  the  digestive 
apparatus  remains  the  same,  the  absorption  of  fat-producing 
materials  in  excess,  implies  defect  in  the  absorption  of  ma- 
terials from  which  the  tissues  are  formed,  and  out  of  which 
young  ones  are  built  up.  Moreover,  this  special 

feeding  with  a  view  to  rapid  and  early  fattening,  continued 
as  it  is  through  generations,  and  accompanied  as  it  is  by 
a  selection  of  individuals  and  varieties  which  fatten  most 
readily,  tends  to  establish  a  modified  constitution,  more  fitted 
for  producing  fat  and  correspondingly-less  fitted  for  producing 
flesh — a  constitution  which,  from  this  relatively- deficient  ab- 
sorption of  nitrogenous  matters,  is  likely  to  become  infertile  ; 
as,  indeed,  these  varieties  generally  become.  Hence, 

no  conclusions  respecting  the  effects  of  high  nutrition,  pro- 
perly so  called,  can  be  drawn  from  cases  of  this  kind.  The 
cases  are,  in  truth,  of  a  kind  that  could  not  exist  but  for 
human  agency.  Under  natural  conditions  no  animal  would 
diet  itself  in  the  way  required  to  produce  such  results.  And 
if  it  did,  its  race  would  quickly  disappear.* 

*  It  is  worth  while  inquiring  whether  unfituess  of  the  food  given  to  them,  is 
aot  the  chief  cause  of  that  sterility  which,  as  Mr.  Darwin  says,  "  is  the  great 
bar  to  the  domestication  of  animals."  He  remarks  that  "when  animals  and 
plants  are  removed  from  their  natural  conditions,  they  are  extrcmelv  liable  to 


162  LAWS   OF   MULTIPLICATION. 

There  is  yet  another  mode  in  which  accumulation  of  fat 
diminishes  fertility.  Even  supposing  it  unaccompanied  by 
a  smaller  absorption  of  nitrogenous  materials,  it  is  still  a 
cause  of  lessening  the  surplus  of  nitrogenous  materials.  For 
the  repair  of  the  motor  tissues  becomes  more  costly.  Fat 
stored-up  is  weight  to  be  carried.  A  creature  loaded  with 
inert  matter  must,  other  things  equal,  consume  a  greater 
amount  of  tissue-forming  substances  for  keeping  its  loco- 
motive apparatus  in  order  ;  and  thus  expending  more  for  self- 
maintenance  can  expend  less  for  race-maintenance.  Abnormal 
plethora  is  thus  antagonistic  to  reproduction  in  a  double  way. 
It  ordinarily  implies  a.  smaller  absorption  of  tissue-forming 
matters,  and  an  increased  demand  on  the  diminished  supply. 
Hence  fertility  decreases  in  a  geometrical  progression. 

The  counter-conclusion  drawn  from  facts  of  this  class,  is, 
then,  due  to  a  misconception  of  their  nature — a  misconception 
arising  partly  from  the  circumstance  that  the  increase  of  bulk 
produced  by  fat  is  somewhat  like  the  increase  of  bulk  which 
growth  of  tissues  causes,  and  partly  from  the  circumstance 
that  abundance  of  good  food  normally  produces  a  certain 
quantity  of  fat,  which,  within  narrow  limits,  is  a  valuable 
store  of  force- evolving  material.  When,  however,  we  limit 
the  phrase  high  nutrition  to  its  proper  meaning — an  abun- 
dance of,  and  due  proportion  among,  all  the  substances  which 
the  organism  needs — we  find  that,  other  things  equal,  fertility 
always  increases  as  nutrition  increases.  And  we  see  that  these 
apparently-exceptional  cases,  are  cases  that  really  show  us  the 
same  thing  ;  since  they  are  cases  of  relative  innutrition. 

have  their  reproductive  systems  seriously  affected."  Possibly  the  relative  or 
absolute  arrest  of  genesis,  is  less  due  to  a  direct  effect  on  the  reproductive  sys- 
tem, than  to  a  changed  nutrition  of  which  the  reproductive  system  most  clearly 
shows  the  results.  The  matters  required  for  forming  an  embryo  are  in  a 
greater  proportion  nitrogenous  than  are  the  matters  required  for  maintain- 
ing an  adult.  Hence,  an  animal  forced  to  live  on  insufficiently-nitrogenized 
food,  may  have  its  surplus  for  reproduction  cut  off,  but  still  have  a  sufficiency 
to  keep  its  own  tissues  in  repair,  and  appear  to  be  in  good  health— meanwhile 
increasing  in  bulk  from  excess  of  the  non-nitrogenous  matters  it  cats. 


CHAPTER  X. 

SPECIALITIES   OF   THESE   RELATIONS. 

|  356.  Tests  of  the  general  doctrines  set  forth  in  preceding 
chapters,  are  afforded  by  organisms  having  modes  of  life  that 
diverge  widely  from  ordinary  modes.  Here,  as  elsewhere, 
aberrant  cases  yield  crucial  proofs. 

If  certain  organisms  are  so  circumstanced  that  highly- 
nutritive  matter  is  supplied  to  them  without  stint,  and  they 
have  nothing  to  do  but  absorb  it,  we  may  infer  that  their 
powers  of  propagation  will  be  enormous. 

If  there  are  classes  of  creatures  that  expend  very  little  for 
self-support  in  comparison  with  allied  creatures,  a  relatively 
extreme  prolificness  may  be  expected  of  them. 

Or  if,  again,  we  find  species  presenting  the  peculiarity 
that  while  some  of  their  individuals  have  much  to  do  and 
little  to  eat,  others  of  their  individuals  have  much  to  eat  and 
little  to  do,  we  may  look  for  great  fertility  in  these  last  and 
comparative  infertility  or  barrenness  in  the  first. 

These  several  anticipations  we  shall  find  completely 
verified. 

§  357.  Plants  which,  like  the  Raffle-siacece,  carry  their  para- 
sitism  to  the  extent  of  living  on  the  juices  they  absorb  from 
other  plants,  exhibit  one  of  these  relations  in  the  vegetal 
kingdom.  In  them  the  organs  for  self-support  being  need- 
less, are  rudimentary ;  and  the  parts  directly  or  indirectly 


464  LAWS   OF   MULTIPLICATION 

concerned  in  the  production  and  distribution  of  germs,  con- 
stitute the  mass  of  the  organism.  That  small  ratio  which 
the  race-preserving  structures  bear  to  the  self-preserving 
structures  in  ordinary  Phsenogams,  is,  in  these  Phrenogams, 
inverted.  A  like  relation  occurs  in  the  common  Dodder. 

There  may  be  added  a  kindred  piece  of  evidence  which  the 
Fungi  present.  Those  of  them  which  grow  on  living  plants, 
repeat  the  above  connection  completely ;  and  those  of  them 
which,  though  not  parasitic,  nevertheless  subsist  on  organized 
materials  previously  elaborated  by  other  plants,  substantially 
repeat  it.  The  spore-producing  part  is  relatively  enormous  ; 
and  the  fertility  is  far  greater  than  that  of  Cryptogams  of  like 
sizes,  which  have  to  form  for  themselves  the  organic  com- 
pounds of  which  they  and  their  germs  consist. 

§  358.  The  same  lesson  is  taught  us  by  animal-parasites. 
Along  with  the  decreased  cost  of  Individuation,  they  similar!}'- 
show  us  an  increased  expenditure  for  Genesis ;  and  they  show 
us  this  in  the  most  striking  manner  where  the  deviation  from 
ordinary  conditions  of  life  is  the  greatest. 

Take,  among  the  Epizoa,  such  an  instance  as  the  Nicothce. 
Belonging  to  the  Entomostraca,  both  males  and  females  of 
this  species  are,  in  their  early  days,  similar  to  their  allies ; 
and  the  males  continue  so  throughout  life.  Each  female, 
however,  presently  fixes  herself  on  the  skin  of  an  aquatic 
animal,  where  she  sits  and  sucks  its  juices,  enlarges  rapidly, 
and  undergoes  an  extreme  distortion,  from  the  growth  of 
the  ovaries.  These,  bulging  out  from  her  sides,  become  lateral 
sacs,  each  of  which  attains  something  like  three  times  her 
size ;  and  then  a  further  distortion  is  produced  by  two  vast 
egg-bags,  severally  larger  than  herself,  which  also  are  formed 
and  become  pendant.  So  that  the  germ-producing  organs  and 
their  contents,  eventually  acquire  a  total  bulk  some  eight  or 
ten  times  that  of  the  rest  of  the  body.  Numerous  species  of 
this  type  and  habit,  repeat  this  relation  between  a  life  of  in- 
action with  high  feeding,  and  an  enormous  rate  of  genesis. 


SPECIALITIES   OF   THESE   RELATIONS.  465 

Entozoa  yield  us  many  examples  of  this  causal  relation, 
raised  to  a  still  higher  degree.  The  Gordius,  or  Hair-worm, 
is  a  creature  which,  finding  its  way  when  young  into  the 
body  of  an  insect,  there  grows  rapidly,  and  afterwards  emerg- 
ing to  breed,  lays  as  many  as  8,000,000  eggs  in  less  than  a  day. 
Similarly  with  the  larger  types  that  infest  the  higher 
animals.  It  has  been  calculated  by  Dr.  Eschricht,  as  quoted 
by  Professor  Owen,  that  there  are  "64,000,000  of  ova  in  the 
mature  female  Ascaris  Lumbricoides,  "  Even  a  still  greater 
fertility  occurs  among  the  cestoid  Entozoa.  Immersed  as  a 
Tape- worm  is  in  nutritive  liquid,  which  it  absorbs  through  its 
integument,  it  requires  no  digestive  apparatus.  The  room 
which  one  would  occupy,  and  the  materials  it  would  use  up, 
are  therefore  available  for  germ-producing  organs,  which 
nearly  fill  each  segment:  each  segment,  sexually  complete  in 
itself,  is  little  else  than  an  enormous  reproductive  system, 
with  just  enough  of  other  structures  to  bind  it  together. 
Remembering  that  the  Tape-worm,  retaining  its  hold,  con- 
tinues to  bud-out  such  segments  as  fast  as  the  fully-developed 
ones  are  cast  off,  and  goes  on  doing  this  as  long  as  the  infested 
individual  lives ;  we  see  that  here,  where  there  is  no  ex- 
penditure, where  the  cost  of  individuation  is  reduced  to  tha 
greatest  extent  while  the  nutrition  is  the  highest  possible, 
the  degree  of  fertility  reaches  its  extreme.  These 

Eiitozoa  yield  us  further  interesting  evidence.  Of  their 
various  species,  most  if  not  all  undergo  passive  migration  from 
animal  to  animal  before  they  become  nature.  Usually,  the 
form  assumed  in  the  body  of  the  first  host,  is  devoid  of  all 
that  part  in  which  the  reproductive  structures  take  their  rise ; 
and  this  part  grows  and  develops  reproductive  structures, 
only  in  some  predatory  animal  to  which  its  first  host  falls  a 
sacrifice.  Occasionally,  however,  the  egg  gives  origin  to  the 
sexual  form  in  the  animal  that  originally  swallowed  it,  but 
the  development  remains  incomplete — there  is  no  sexual 
genesis,  no  formation  of  eggs  in  the  rudimentary  segments. 
That  these  may  become  fertile,  it  is  needful,  as  before,  for  the 


lt)6  LAWS   OF    MULTIPLICATION. 

containing  animal  to  be  devoured;  so  that  the  imperfect  Tape 
worm  may  find  its  way  into  the  intestine  of  a  higher  animal. 
Thus  the  Bothriocephalus  solid  as,  found  in  the  abdominal  cavity 
of  the  Stickleback,  is  barren  while  it  remains  there  ;  but  if  the 
Stickleback  is  eaten  by  a  Water-fowl,  the  reproductive  system 
of  the  transferred  Botlirioccphnhts  becomes  developed  and 
active.  So,  too,  a  kind  of  Tape- worm  which  remains  infertile 
while  in  the  intestine  of  a  Mouse,  becomes  fertile  in  the  in- 
testine of  a  Cat  that  devours  the  mouse.  May  we  not  regard 
these  facts  as  again  showing  the  dependence  of  fertility  on 
nutrition  ?  Barrenness  here  accompanies  conditions  unfavour- 
able to  the  absorption  of  nutriment ;  and  it  gives  way  to 
fecundity  where  nutriment  is  large  in  quantity  and  superior 
in  quality. 

§  359.  Extremely  significant  are  those  cases  of  partial 
reversion  to  primitive  forms  of  genesis,  that  occur  under 
special  conditions  in  some  of  the  higher  Anmdom.  I  refer  to 
the  pseudo-parthenogenesis  and  metagenesis  in  Insects. 

Under  what  conditions  do  the  Aphides  exhibit  this  strange 
deviation  from  the  habits  of  their  order  ?  Why  among  them 
should  imperfect  females  produce,  agamically,  others  like 
themselves,  generation  after  generation,  with  great  rapidity  ? 
There  is  the  obvious  explanation  that  they  get  plenty  of 
easily-assimilated  food  without  exertion.  Piercing  the  tender 
coats  of  young  shoots,  they  sit  and  suck — appropriating  the 
nitrogenous  elements  of  the  sap  and  ejecting  its  saccharine 
matter  as  "  honey  dew."  Along  with  a  sluggishness 
strongly  contrasted  with  the  activity  of  their  allies — along 
with  a  very  low  rate  of  consumption  and  a  correlative  degra- 
dation of  structure ;  we  have  her3  a  retrogression  to  asexual 
genesis,  and  a  greatly-increased  rate  of  multiplication. 

The  recently- discovered  instance  of  internal  metagenesis 
in  the  maggots  of  certain  Flies  has  a  like  meaning.  In- 
credible as  it  at  first  seemed  to  naturalists,  it  is  now  proved  that 
the  Cecydomia-la.Y\9,  develops  in  its  interior  a  brood  of  larvae 


SPECIALITIES    OF    THESE    RELATIONS.  467 

of  like  structure  with  itself.  In  this  case,  as  in  the  last,  abun- 
dant food  is  combined  with  low  expenditure.  These  larvae  are 
found  in  such  habitats  as  the  refuse  of  beet-root-sugar  fac- 
tories— masses  of  nitrogenous  debris  remaining  after  the 
extraction  of  the  saccharine  matter.  Each  larva  has  a 
practically -unlimited  supply  of  sustenance  imbedding  it  on 
all  sides. 

It  is  true  that  some  other  maggots,  as  those  of  the  Flesh-fly, 
are  similarly,  or  still  better,  circumstanced ;  and,  it  may  be 
said,  ought  therefore  to  have  the  same  habit.  But  this  does 
not  necessarily  follow.  Survival  of  the  fittest  will  determine 
whether  such  specially-favourable  conditions  result  in  the 
aggrandisement  of  the  individual  or  in  the  multiplication  of 
the  race.  And  in  the  case  of  the  Flesh-fly,  there  is  a  reason 
why  greater  individuation  rather  than  more  rapid  genesis 
will  occur.  For  a  decomposing  animal  body  lasts  so  short  a 
time,  that  were  Flesh-fly  laj-voe  to  multiply  agamically,  the 
second  generation  would  die  from  the  disappearance  of  their 
food.  Hence,  individuals  in  which  the  excessive  nutrition 
led  to  internal  metagenesis,  would  leave  no  posterity ;  and 
natural  selection  would  establish  the  variety  in  which  greater 
growth  resulted.  All  which  the  argument  requires  is,  that 
when  such  reversion  to  agamogenesis  does  take  place,  it  shall 
be  where  the  food  is  unusually  abundant  and  the  expenditure 
unusually  small ;  and  this  the  cases  instanced  go  to  show. 

§  360.  The  physiological  lesson  taught  us  by  Bees  and 
Ants,  not  quite  harmonizing  with  the  moral  lesson  they  are 
supposed  to  teach,  is  that  highly-fed  idleness  is  favourable  to 
fertility,  and  that  excessive  industry  has  barrenness  for  its 
concomitant. 

The  egg  of  a  Bee  develops  into  a  small  barren  female  or 
into  a  large  fertile  female,  according  to  the  supply  of  food 
given  to  the  larva  hatched  from  it.  We  here  see  that  the 
germ-producing  action  is  an  overflow  of  the  surplus  remain- 
ing after  completion  of  the  individual ;  and  that  the  lower 


468  LAWS   OF   MULTIPUCATION. 

feeding  which  the  larva  of  a  working  Bee  has,  results  in  a 
dwarfing  of  the  adult  and  an  arrested  development  of  the 
generative  organs.  Further,  we  have  the  fact  that  the  con- 
dition under  which  the  perfect  female,  or  mother-Bee,  goes 
on,  unlike  insects  in  general,  laying  eggs  continuously,  is 
that  she  has  plenty  of  food  brought  to  her,  is  kept  warm,  and 
goes  through  no  considerable  exertion.  While,  contrariwise, 
it  is  to  be  noted  that  the  infertility  of  the  workers,  is  asso- 
ciated with  the  ceaseless  labour  of  bringing  materials  for  the 
combs  and  building  them,  as  well  as  the  labour  of  feeding 
the  queen,  the  larvae,  and  themselves. 

Ants,  and  especially  some  of  the  tropical  kinds,  show 
us  these  relations  in  an  exaggerated  form.  The  differ- 
ence of  bulk  between  the  fecund  and  infecund  females  is 
immensely  greater.  The  mother- Ant  has  the  reproductive 
system  so  enormously  developed,  that  the  remainder  of  her 
body  is  relatively  insignificant.  Entirely  incapable  of  loco- 
motion, she  is  unable  to  deposit  her  eggs  in  the  places  where 
they  are  to  be  hatched ;  so  that  they  have  to  be  carried  away 
by  the  workers  as  fa'st  as  they  are  extruded.  Her  life  is  thus 
reduced  substantially  to  that  of  a  parasite — an  absorption  of 
abundant  food  supplied  gratis,  a  total  absence  of  expendi- 
ture, and  a  consequent  excessive  rate  of  genesis.  "  The 
queen-ant  of  the  African  Termites  lays  80,000  eggs  in  twenty- 
four  hours."  • 

§  361.  It  may  be  needful  to  say  that  these  exceptional 
relations  cannot  be  ascribed  to  the  assigned  causes  acting 
alone.  The  extreme  fertility  which,  among  parasites  and 
social  insects,  accompanies  extremely  high  feeding,  and  an 
expenditure  reduced  nearly  to  zero,  presupposes  typical  struc- 
tures and  tendencies  of  suitable  kinds;  and  these  are  not 
directly  accounted  for.  On  creatures  otherwise  organized, 
unlimited  supplies  of  food  and  total  inactivity  are  not  fol- 
lowed by  such  results.  There  of  course  requires  a  consti- 
tution fitted  to  the  special  conditions ;  and  the  evolution  of 


SPECIALITIES   OF   THESE   RELATIONS.  4G9 

this  cannot  be  due  simply  to  plethora  joined  with  rest.  These 
cases  are  given  as  illustrating  the  conditions  under  which 
extreme  exaltations  of  fertility  become  possible.  Their  mean- 
ings, thus  limited,  are  clear,  and  completely  to  the  point.  We 
see  in  them  that  the  devotion  of  nutriment  to  race-preserva- 
tion, is  carried  furthest  where  the  cost  of  self-preservation 
is  reduced  to  a  minimum;  and,  conversely,  that  nothing 
is  devoted  directly  to  race- preservation  by  individuals  on 
which  falls  an  excessive  expenditure  for  self-preservation  and 
preservation  of  other's  offspring. 


CHAPTER  XT. 

INTERPRETATION  AND  QUALIFICATION. 

§  362.  Considering  the  difficulties  of  inductive  verification, 
we  have,  I  think,  as  clear  a  correspondence  between  the 
a  priori  and  d  posteriori  conclusions,  as  can  be  expected.  The 
many  factors  co-operating  to  bring  about  the  result  in  every 
case,  are  so  variable  in  their  absolute  and  relative  amounts, 
that  we  can  rarely  disentangle  the  effect  of  each  one ;  and 
have  usually  to  be  content  with  qualified  inferences.  Though 
in  the  mass,  organisms  show  us  an  unmistakable  relation 
between  great  size  and  small  fertility ;  yet  special  comparisons 
among  them  are  nearly  always  partially  vitiated  by  differ- 
ences of  structure,  differences  of  nutrition,  differences  of 
expenditure.  Though  it  is  beyond  question  that  the  more 
complex  organisms  are  the  less  prolific  ;  yet  as  complexity  has 
a  certain  general  connexion  with  bulk,  and  in  animals  with 
expenditure,  we  cannot  often  identify  its  results  as  inde- 
pendent of  these.  And,  similarly,  though  the  creatures  that 
waste  much  matter  in  producing  motion,  sensible  and  insen- 
sible, have  lower  rates  of  multiplication  than  those  which 
waste  less ;  yet,  as  the  creatures  which  waste  much  are 
generally  larger  and  more  complex,  we  are  again  met  by  an 
obstacle  which  limits  our  comparisons,  and  compels  us  to 
accept  conclusions  less  definite  than  are  desirable. 

Such  difficulties  arise,  however,  only  when  we  endeavour, 
BS  in  foregoing  chapters,  to  prove  the  inverse  variation 


INTERPRETATION'    AND    QUALIFICATION.  471 

between  Genesis  and  each  separate  element  of  Individuation 
• — growth,  development,  activity.  We  are  scarcely  at  all 
hampered  by  qualifications  when,  from  contemplating  these 
special  relations,  we  return  to  the  general  relation.  The 
antagonism  between  Individuation  and  Genesis,  is  shown  by 
all  the  facts  that  have  been  grouped  under  each  head.  We 
have  seen  that  in  ascending  from  the  lowest  to  the  highest 
types,  there  is  a  decrease  of  fertility  so  great  as  to  be  abso- 
lutely inconceivable,  and  even  inexpressible  by  figures  ;  and 
whether  the  superiority  of  type  consists  in  relative  largeness, 
in  greater  complexity,  in  higher  activity,  or  in  some  or  all  of 
these  combined,  matters  not  to  the  ultimate  inference.  The 
broad  fact,  enough  for  us  here,  is  that  organisms  in  which 
the  integration  and  differentiation  of  matter  and  motion  have 
been  carried  furthest,  are  those  in  which  the  rate  of  multipli- 
cation has  fallen  lowest.  How  much  of  the  decline  of  repro- 
ductive power  is  due  to  the  greater  integration  of  matter, 
how  much  to  its  greater  differentiation,  how  much  to  the 
larger  amounts  of  integrated  and  differentiated  motions  gene- 
rated, it  may  be  impossible  to  say;  and  it  is  not  needful  to 
say.  These  are  all  elements  of  a  higher  degree  of  life,  an 
augmented  ability  to  maintain  the  organic  equilibrium  amid 
environing  actions — an  increased  power  of  self-preservation  ; 
and  we  find  their  invariable  accompaniment  to  be,  a  dimi- 
nished expenditure  of  matter,  or  motion,  or  both,  in  race- 
preservation. 

In  brief,  then,  examination  of  the  evidence  shows  that 
there  does  exist  that  relation  which  we  inferred  must  exist. 
Arguing  from  general  data,  we  saw  that  for  the  maintenance 
of  a  species,  the  ability  to  produce  offspring  must  be  great, 
in  proportion  as  the  ability  of  the  individuals  to  contend  witk 
destroying  forces  is  small;  and  conversely.  Arguing  from 
other  general  data,  we  saw  that,  derived  as  the  self-sustain- 
ing and  race-sustaining  forces  are  from  a  common  stock  of 
force,  it  necessarily  happens  that,  other  things  equal,  increase 
of  one  involves  decrease  of  the  other.  And  then,  turning 


472  LAWS   OF    MULTIPLICATION. 

(o  special  facts,  we  have  found  that  this  inverse  variation  ia 
clearly  traceable  throughout  both  the  animal  and  vegetal 
kingdoms.  We  may  therefore  set  it  down  as  a  law,  that 
everv  higher  degree  of  organic  evolution,  has  for  its  con- 
comitant a  lower  degree  of  that  peculiar  organic  dissolution 
which  is  seen  in  the  production  of  new  organisms. 

§  363.  Something  remains  to  be  said  in  reply  to  the  in- 
quiry— how  is  the  ratio  between  Individuation  and  Genesis 
established  in  each  case  ?  This  inquiry  has  been  but  partially 
answered  in  the  course  of  the  foregoing  argument. 

All  specialities  of  the  reproductive  process  are  due  to  the 
natural  selection  of  favourable  variations.  Whether  a  creature 
lays  a  few  large  eggs  or  many  small  ones  equal  in  weight  to 
the  few  large,  is  not  determined  by  any  physiological  neces- 
sity :  here  the  only  assignable  cause  is  the  survival  of  varieties 
in  which  the  matter  devoted  to  reproduction,  happens  to  be 
divided  into  portions  of  such  size  and  number  as  most  to 
favour  multiplication.  Whether  in  any  case  there  are 
frequent  small  broods  or  larger  broods  at  longer  intervals, 
depends  wholly  on  the  constitutional  peculiarity  that  lias 
arisen  from  the  dying  out  of  families  in  which  the  sizes  and 
intervals  of  the  broods  were  least  suited  to  the  conditions  of 
life.  Whether  a  species  of  animal  produces  many  offspring 
of  which  it  takes  no  care  or  a  few  of  which  it  takes  much 
care — that  is,  whether  its  reproductive  surplus  is  laid  out 
wholly  in  germs  or  partly  in  germs  and  partly  in  labour  on 
their  behalf — must  have  been  decided  by  that  moulding  of 
constitution  to  conditions,  slowly  effected  through  the  more 
frequent  preservation  of  descendants  from  those  whose  re- 
productive habits  were  best  adapted  to  the  circum- 
stances of  the  species.  Given  a  certain  surplus  available 
for  race-preservation,  and  it  is  clear  that  by  indirect 
equilibration  only,  can  there  be  established  the  more  or 
less  peculiar  distribution  of  this  surplus  which  we  see  in 
each  case.  Obviouslv,  too,  survival  of  the  fittest 


IJiTEKl'llErATIOX    A.XD    QUALIFICATION.  473 

has  a  share  in  determining  the  proportion  between  the 
amount  of  matter  that  goes  to  Iiidividuation  and  the  amount 
that  goes  to  Genesis.  Whether  the  interests  of  the  species 
are  most  subserved  by  a  higher  evolution  of  the  individual 
joined  with  a  diminished  fertility,  or  by  a  lower  evolution  of 
the  individual  joined  with  an  increased  fertility,  are  ques- 
tions ever  being  experimentally  answered.  If  the  more- 
developed  and  less-prolific  variety  has  a  greater  number  of 
survivors,  it  becomes  established  and  predominant.  If,  con- 
trariwise, the  conditions  of  life  being  simple,  the  larger  or 
more-organized  individuals  gain  nothing  by  their  greater  size 
or  better  organization  ;  then  the  greater  fertility  of  the  less 
evolved  ones,  will  insure  to  their  descendants  an  increasing 
predominance. 

But  direct  equilibration  all  along  maintains  the  limits 
within  which  indirect  equilibration  thus  works.  The 
necessary  antagonism  we  have  traced,  rigidly  restricts  the 
changes  that  natural  selection  can  produce,  under  given  con- 
ditions, in  either  direction.  A  greater  demand  for  Individua- 
tion,  be  it  a  demand  caused  by  some  spontaneous  variation  or 
by  an  adaptive  increase  of  structure  and  function,  inevitably 
diminishes  the  supply  for  Genesis ;  and  natural  selection 
cannot,  other  things  remaining  the  same,  restore  the  rate  of 
Genesis  while  the  higher  Individuation  is  maintained.  Con- 
versely, survival  of  the  fittest,  acting  on  a  species  that  has, 
by  spontaneous  variation  or  otherwise,  become  more  prolific, 
cannot  again  raise  its  lowered  Individuation,  so  long  as  every- 
thing else  continues  constant. 

§  364.  Here,  however,  a  qualification  must  be  made.  It 
was  parenthetically  remarked  in  §  327  that  the  inverse  varia- 
tion between  Individuation  and  Genesis  is  not  exact ;  and  it 
was  hinted  that  a  slight  modification  of  statement  would  be 
requisite  at  a  more  advanced  stage  of  the  argument.  We 
have  now  reached  the  proper  place  for  specifying  this 
modification. 


174  LAWS    OF    MULTIPLICATION. 

Each  increment  of  evolution  entails  a  decrement  of  re- 
production that  is  not  accurately  proportionate,  but  somewhat 
less  than  proportionate.  The  gain  in  the  one  direction  is  not 
wholly  canceled  by  a  loss  in  the  other  direction,  but  only 
partially  canceled  :  leaving  a  margin  of  profit  to  the  species. 
Though  augmented  power  of  self-maintenance  habitually 
necessitates  diminished  power  of  race- propagation,  yet  the 
product  of  the  two  factors  is  greater  than  before  ;  so  that  the 
forces  preservative  of  race  become,  thereafter,  in  excess  of  the 
forces  destructive  of  race,  and  the  race  spreads.  We  shall 
soon  see  why  this  happens. 

Each  advance  in  evolution  implies  an  economy.  That  any 
increase  in  bulk,  or  structure,  or  activity,  may  become  esta- 
blished, the  life  of  the  organism  must  be  to  some  extent 
facilitated  by  the  change — the  cost  of  self-support  must  be, 
on  the  average,  reduced.  If  the  greater  complexity,  or  the 
larger  size,  or  the  more  agile  movement,  entails  on  the  in- 
dividual an  outlay  that  is  not  repaid  in  food  more-easily 
obtained,  or  danger  more-easily  escaped  ;  then  the  individual 
will  be  at  a  relative  disadvantage,  and  its  diminished  posterity 
will  disappear.  If  the  extra  outlay  is  but  just  made  good 
by  the  extra  advantage,  the  modified  individual  will  not  sur- 
vive longer,  or  leave  more  descendants,  than  the  unmodified 
individuals.  Consequently,  it  is  only  when  the  expense  of 
greater  individuation  is  out-balanced  by  a  subsequent  saving, 
that  it  can  tend  to  subserve  the  preservation  of  the  indi- 
vidual; or,  by  implication,  the  preservation  of  the  race. 
The  vital  capital  invested  in  '  the  alteration  must  bring 
a  more  than  equivalent  return.  A  few  instances 

will  show  that,  whether  the  change  results  from  direct 
equilibration  or  from  indirect  equilibration,  this  must  happen. 
Suppose  a  creature  takes  to  performing  some  act  in  an  un- 
usual way — leaps  where  ordinarily  its  kindred  crawl,  eludes 
pursuit  by  diving  instead  of,  like  others  of  its  kind,  by  swim- 
ming along  the  surface,  escapes  by  doubling  instead  of  by  sheei 
speed.  Clearly,  perseverance  in  the  modified  habit  will,  other 


INTERPRETATION    AND   QUALIFICATION.  47<5 

things  equal,  imply  that  it  takes  less  effort.  The  creature's 
sensations  will  ever  prompt  desistance  from  the  more  laborious 
course;  and  hence  a  congenital  habit  is  not  likely  to  be 
diverged  from  unless  an  economy  of  force  is  achieved  by  the 
divergence.  Assuming,  then,  that  the  new  method  has  no 
advantage  over  the  old  in  directly  diminishing  the  chances 
of  death,  the  establishment  of  it,  and  of  the  structural 
complications  involved,  nevertheless  implies  a  physiological 
gain.  Suppose,  again,  that  an  animal  takes  to  some 
abundant  food  previously  refused  by  its  kind.  It  is  likely  to 
persist  only  if  that  the  comparative  ease  in  obtaining  this 
food,  more  than  compensates  for  any  want  of  adaptation  to  its 
digestive  organs ;  so  that  superposed  modifications  of  the 
digestive  organs  are  likely  to  arise  only  when  an  average 
economy  results.  What  now  must  be  the  influence 

on  the  creature's  system  as  a  whole  ?  Diminished  expenditure 
in  any  direction,  or  increased  nutrition  however  effected, 
will  leave  a  greater  surplus  of  materials.  The  animal  will  be 
physiologically  richer.  Part  of  its  augmented  wealth  will  go 
towards  its  own  greater  individuation — its  size,  or  its  strength, 
or  both,  will  increase;  while  another  part  will  go  towards 
more  active  genesis.  Just  as  a  state  of  plethora  directly 
produced  enhances  fertility ;  so  will  such  a  state  indirectly 
produced. 

In  another  way,  the  same  thing  must  result  from  those 
additions  to  bulk  or  complexity  or  activity  that  are  due  to 
survival  of  the  fittest.  Any  change  which  prolongs  individual 
life,  will,  other  things  remaining  the  same,  further  the  pro- 
duction of  offspring.  Even  when  it  is  not,  like  the  foregoing, 
a  means  of  economizing  the  forces  of  the  individual,  still,  if  it 
increases  the  chances  of  escaping  destruction,  it  increases  the 
chances  of  leaving  posterity.  Any  further  degree  of  evolution, 
therefore,  will  be  so  established  only  where  the  cost  of  it  is 
more  than  repaid ;  part  of  the  gain  being  shown  in  the 
lengthened  life  of  the  individual,  and  part  in  the  greater 
production  of  other  individuals. 


476  LAWS   OF    MULTIPLICATION. 

We  have  here  the  solution  of  various  minor  anomalies  l>v 
which  the  inverse  variation  of  Individuation  and  Genesis  is 
obscured.  Take  as  an  instance  the  fertility  of  the  Blackbird 
as  compared  with  that  of  the  Linnet.  Both  birds  lay  five  eggs, 
and  both  usually  have  two  broods.  Yet  the  Blackbird  is  far 
the  larger  of  the  two ;  and  ought,  according  to  the  general 
law,  to  be  much  less  prolific.  What  causes  this  noncon- 
formity? We  shall  find  an  answer  in  their  respective  foods 
and  habits.  Except  during  the  time  that  it  is  rearing  its 
young,  the  Linnet  collects  only  vegetal  food — lives  during 
the  winter  on  the  seeds  it  finds  in  the  fields,  or,  when  hard 
pressed,  picks  up  around  farms ;  and  to  obtain  this  spare 
diet  is  continually  flying  about.  The  result,  if  it  survives  the 
frost  and  snow,  is  a  considerable  depletion;  and  it  recovers 
its  condition  only  after  some  length  of  spring  weather.  The 
Blackbird,  on  the  other  hand,  is  omnivorous :  while  it  eats 
grain  and  fruit  when  they  come  in  its  way,  it  depends  largely 
on  animal  food.  It  cuts  to  pieces  and  devours  the  dew- worms 
which,  morning  and  evening,  it  finds  on  the  surface  of  a  lawn, 
and,  even  discovering  where  they  are,  unearths  them ;  it 
swallows  slugs,  and  breaking  snail-shells,  either  with  its  beak 
or  by  hammering  them  against  stones,  tears  out  their  tenants; 
and  it  eats  beetles  and  larvae.  Thus  the  strength  of  the 
Blackbird  opens  to  it  a  store  of  good  food,  much  of  which  is 
inaccessible  to  so  small  and  weak  a  bird  as  a  Linnet — a  store 
especially  helpful  to  it  during  the  cold  months,  when  the 
hybernating  Snails  in  hedge-bottoms  yield  it  abundant  pro- 
vision. The  result  is  that  the  Blackbird  is  ready  to  breed 
very  early  in  spring ;  and  is  able  during  the  summer  to  rear 
a  second,  and  sometimes  oven  a  third,  brood.  Here,  then,  a 
higher  degree  of  Individuation  secures  advantages  so  great, 
as  to  much  more  than  compensate  its  cost :  it  is  not  that  the 
decline  of  Genesis  is  less  than  proportionate  to  the  increase  of 
Individuation,  but  there  is  no  decline  at  all.  Com- 

parison of  the  Eat  with  the  Mouse  yields  a  parallel  result. 
Though  they  differ  greatly  in  size,  yet  the  one  is  as  prolific 


IX  lERPKETATIOX    AXD    QUALIFICATION.  477 

as  the  other.  This  absence  of  difference  cannot  be  ascribed 
to  their  unlike  degrees  of  activity.  We  must  seek  its  cause  in 
some  facility  of  living  secured  to  the  Bat  by  its  greater  intel- 
ligence, greater  power  and  courage,  greater  ability  to  utilize 
what  it  finds.  The  Rat  is  notoriously  cunning ;  and  its 
cunning  gives  success  to  its  foraging  expeditions.  It  is  not, 
like  the  Mouse,  limited  mainly  to  vegetal  food ;  but  while  it 
eats  grain  and  beans  like  the  Mouse,  it  also  eats  flesh  and 
carrion,  devours  young  poultry  and  eggs.  The  result  is  that, 
without  a  proportionate  increase  of  expenditure,  it  gets  a  far 
larger  supply  of  nourishment  than  the  Mouse  ;  and  this  rela- 
tive excess  of  nourishment  makes  possible  a  large  size  without 
a  smaller  rate  of  multiplication.  How  clearly  this  is  the 
cause,  we  see  in  the  contrast  between  the  common  Rat  and 
the  Water-Rat.  While  the  common  Rat  has  habitually 
several  broods  a  year  of  from  10  to  12  each,  the  Water-Rat, 
though  somewhat  smaller,  has  but  5  or  6  in  a  brood,  and  but 
one  brood,  or  sometimes  two  broods,  a-year.  But  the  Water- 
Rat  lives  on  vegetal  food — lacks  all  that  its  bold,  sagacious, 
omnivorous  congener,  gains  from  the  warmth  as  well  as  the 
abundance  which  men's  habitations  yield. 

The  inverse  variation  of  Individuation  and  Genesis  is, 
therefore,  but  approximate.  Recognizing  the  truth  that 
every  increment  of  evolution  which  is  appropriate  to  the 
circumstances  of  an  organism,  brings  an  advantage  somewhat 
in  excess  of  its  cost ;  we  see  the  general  law,  as  more  strictly 
stated,  to  be  that  Genesis  decreases  not  quite  so  fast  as  In- 
dividuation increases.  Whether  the  greater  Individuation 
takes  the  form  of  a  larger  bulk  and  accompanying  access  of 
strength  ;  whether  it  be  shown  in  higher  speed  or  agility ; 
whether  it  consists  in  a  modification  of  structure  that 
facilitates  some  habitual  movement,  or  in  a  visceral  change 
that  helps  to  utilize  better  the  absorbed  aliment ;  the  ultimate 
effect  is  identical.  There  is  either  a  more  economical  per- 
formance of  the  same  actions,  internal  or  external,  or  there 
is  a  securing  of  greater  advantages  by  modified  actions,  which 


478  LAWS    OF    MULTIPLICATION. 

cost  no  more,  or  have  an  increased  cost  less  than  the  in- 
creased gain.  In  any  case,  the  result  is  a  greater  surplus  of 
vital  capital ;  part  of  which  goes  to  the  aggrandisement 
of  the  individual,  and  part  to  the  formation  of  new  in- 
dividuals. While  the  higher  tide  of  nutritive  matters, 
everywhere  filling  the  parent-organism,  adds  to  its  power  of 
self-maintenance,  it  also  causes  a  reproductive  overflow  larger 
than  before. 

Hence  every  type  that  is  best  adapted  to  its  conditions, 
which  on  the  average  means  every  higher  type,  has  a  rate  of 
multiplication  that  insures  a  tendency  to  predominate. 
Survival  of  the  fittest,  acting  alone,  is  ever  replacing  in- 
ferior species  by  superior  species.  But  beyond  the  longer 
urvival,  and  therefore  greater  chance  of  leaving  offspring, 
which  superiority  gives,  we  see  here  another  way  in  which 
the  spread  of  the  superior  is  insured.  Though  the  more- 
ovolved  organism  is  the  less  fertile  absolutely,  it  is  the  more 
fertile  relatively. 


CHAPTER  XII. 

MULTIPLICATION    OF    THE    HUMAN    RACE. 

§  365.  The  relative  fertility  of  Man  considered  as  a  species, 
and  those  changes  in  Man's  fertility  which  occur  under 
changed  conditions,  must  conform  to  the  laws  which  we  have 
traced  thus  far.  As  a  matter  of  course,  the  inverse  variation 
between  Individuation  and  Genesis,  holds  of  him  as  of  all 
other  organized  beings.  His  extremely  low  rate  of  multipli- 
cation— far  below  that  of  all  terrestrial  Mammals  except  the 
Elephant,  (which  though  otherwise  less  evolved,  is,  in  extent 
of  integration,  moro  evolved) — we  shall  recognize  as  the 
necessary  concomitant  of  his  much  higher  evolution.  And 
the  causes  of  increase  or  decrease  in  his  fertility,  special  or 
general,  temporary  or  permanent,  we  shall  expect  to  find  in 
those  changes  of  bulk,  of  structure,  or  of  expenditure,  which 
we  have  in  all  other  cases  seen  associated  with  such  effects. 

In  the  absence  of  detailed  proof  that  these  parallelisms 
exist,  it  might  suffice  to  contemplate  the  several  communities 
between  the  reproductive  function  in  human  beings  and  other 
beings.  I  do  not  refer  simply  to  the  fact  that  genesis  pro- 
ceeds in  a  similar  manner  ;  but  I  refer  to  the  similarity  of 
the  relation  between  the  generative  function  and  the  func- 
tions that  have  for  their  joint  end  the  preservation  of  the 
individual.  In  Man,  as  in  other  creatures  that  expend  much, 
genesis  commences  only  when  growth  and  development  are 
declining  in  rapidity  and  approaching  their  termination. 
Among  the  higher  organisms  in  general,  the  reproductive 
VOL.  II.  21 


480  LAWS   OF   MULTIPLICATION. 

activit)-,  continuing  during  the  prime  of  life,  ceases  when  the 
vigour  declines,  leaving  a  closing  period  of  infertility ;  and  in 
like  manner  among  ourselves,  barrenness  supervenes  when 
middle  age  brings  the  surplus  vitality  to  an  end.  So,  too, 
it  is  found  that  in  Man,  as  in  beings  of  lower  orders,  there  is 
a  period  at  which  fecundity  culminates.  In  §  341,  facts  were 
cited  showing  that  at  the  commencement  of  the  reproductive 
period,  animals  bear  fewer  offspring  than  afterwards ;  and 
that  towards  the  close  of  the  reproductive  period,  there  is  a 
decrease  in  the  number  produced.  In  like  manner  it  is  shown 
by  the  tables  of  Dr.  Duncan's  recent  work,  that  the  fecundity 
of  women  increases  up  to  the  age  of  about  25  years ;  and 
continuing  high  with  but  slight  diminution  till  after  30, 
then  gradually  wanes.  It  is  the  same  with  the  sizes  and 
weights  of  offspring.  Infants  born  of  women  from  25  to  29 
years  of  age,  are  both  longer  and  heavier  than  infants  born 
of  younger  or  older  women ;  and  this  difference  has  the  same 
implication  as  the  greater  total  weight  of  the  offspring  pro- 
duced at  a  birth,  during  the  most  fecund  age  of  a  pluriparous 
animal.  Once  more,  there  is  the  fact  that  a  too-early  bearing 
of  young  produces  on  a  woman  the  same  injurious  effects  as 
on  an  inferior  creature— an  arrest  of  growth  and  an  enfeeble- 
ment  of  constitution. 

Considering  these  general  and  special  parallelisms,  we 
might  safely  infer  that  variations  of  human  fertility  conform 
to  the  same  laws  as  do  variations  of  fertility  in  general. 
But  it  is  not  needful  to  content  ourselves  with  an  implication. 
Evidence  is  assignable  that  what  causes  increase  or  decrease 
of  genesis  in  other  creatures,  causes  increase  or  decrease  of 
genesis  in  Man.  It  is  true  that,  even  more  than  hitherto,  our 
reasonings  are  beset  by  difficulties.  So  numerous  are  the 
inequalities  in  the  conditions,  that  but  few  unobjectionable 
comparisons  can  be  made.  The  human  races  differ  consider- 
ably in  their  sizes,  and  notably  in  their  degrees  of  cerebral 
development.  The  climates  they  inhabit  entail  on  them 
widely  different  consumptions  of  matter  for  maintenance  of 


MULTIPLICATION   OF   THE    HUMAN    KACE.  481 

temperature.  Both  in  their  qualities  and  quantities,  the 
foods  they  live  on  are  unlike  ;  and  the  supply  is  here  regular 
and  there  very  irregular.  Their  expenditures  in  bodily  action 
are  extremely  unequal ;  and  even  still  more  unequal  are 
their  expenditures  in  mental  action.  Hence  the  factors, 
varying  so  much  in  their  amounts  and  combinations,  can 
scarcely  ever  have  their  respective  effects  identified.  Never- 
theless there  are  u  few  comparisons,  the  results  of  which  may 
withstand  criticism. 

§  366.  The  increase  of  fertility  caused  by  a  nutrition  that 
is  greatly  in  excess  of  the  expenditure,  is  to  be  detected  by 
contrasting  populations  of  the  same  race,  or  allied  races, 
one  of  which  obtains  good  and  abundant  sustenance  much 
more  easily  than  the  other.  Three  cases  may  here  be  set 
down. 

The  traveller  Barrow,  describing  the  Cape- Boors,  says : — 
"  Unwilling  to  work  and  unable  to  think,"  *  *  *  "  indulging 
to  excess  in  the  gratification  of  every  sensual  appetite,  the 
African  peasant  grows  to  an  unwieldy  size ; "  and  respecting 
the  other  sex,  he  adds — "  the  women  of  the  African  peasantry 
lead  a  life  of  the  most  listless  inactivity."  Then,  after  illus- 
trating these  statements,  he  goes  on  to  note  "  the  prolific 
tendency  of  all  the  African  peasantry.  Six  or  seven  children 
in  a  family  are  considered  as  very  few ;  from  a  dozen  to 
twenty  are  not  uncommon."  The  native  races  of 

this  region  yield  evidenca  to  the  same  effect.  JSpeaking  of 
the  cruelly- used  Hottentots  (he  is  writing  sixty  years  ago), 
who,  while  they  are  poor  and  ill- fed,  have  to  do  all  the  work 
for  the  idle  Boors,  Barrow  says  that  they  "  seldom  have  more 
than  two  or  three  children  ;  and  many  of  the  women  are 
barren."  This  unusual  infertility  stands  in  remarkable  con- 
trast with  the  unusual  fertility  of  the  Kaffirs,  of  whom  he 
afterwards  gives  an  account.  Rich  in  cattle,  leading  easy 
lives,  and  living  almost  exclusively  on  animal  food  (chiefly 
milk  with  occasional  flesh),  these  people  were  then  reputed 


482  LAWS   OF    MULTIPLICATION. 

to  have  a  very  high  rate  of  multiplication.  Barrow  writes  : — 
"  They  are  said  to  be  exceedingly  prolific ;  that  twins  are 
almost  as  frequent  as  single  births,  and  that  it  is  no  un- 
common thing  for  a  woman  to  have  three  at  a  time."  Pro- 
bably both  these  statements  are  in  excess  of  the  truth ;  but 
there  is  room  for  large  discounts  without  destroying  the 
extreme  difference.  A  third  instance  is  that  of  the 

French  Canadians.  "Nous  sommes  tcrribles  pour  les  enfants  !" 
observed  one  of  them  to  Prof.  Johnston ;  who  tells  us  that 
the  man  who  said  this  "  was  one  of  fourteen  children — was 
himself  the  father  of  fourteen,  and  assured  me  that  from 
eight  to  sixteen  was  the  usual  number  of  the  farmers' 
families.  He  even  named  one  or  two  women  who  had 
brought  their  husbands  five-and-twenty,  and  threatened  '  le 
vingt-sixi&me  pour  le  pretrc.'  "  From  these  large"  families, 
joined  with  the  early  marriages  and  low  rate  of  mortality,  it 
results  that,  by  natural  increase,  "  there  are  added  to  the 
French-Canadian  population  of  Lower  Canada  four  persons 
for  every  one  that  is  added  to  the  population  of  England." 
Now  these  French-Canadians  are  described  by  Prof.  Johnston 
as  home-loving,  contented,  unenterprising;  and  as  living  in 
a  region  where  "  land  and  subsistence  are  easily  obtained." 
Very  moderate  industry  brings  to  them  liberal  supplies  of 
necessaries  ;  and  they  pass  a  considerable  portion  of  the  year 
in  idleness.  Hence  the  cost  of  Individuation  being  much 
reduced,  the  rate  of  Genesis  is  much  increased.  That  this 
uncommon  fertility  is  not  due  to  any  direct  influence  of  the 
locality,  is  implied  by  the  fact  that  along  with  the  "  restless, 
discontented,  striving,  burning  energy  of  their  Saxon  neigh- 
bours "  no  such  rate  of  multiplication  is  observed  ;  while 
further  south,  where  the  physical  circumstances  are  more 
favourable  if  anything,  the  Anglo-Saxons,  leading  lives  of 
excessive  activity,  have  a  fertility  below  the  average.  And 
that  the  peculiarity  is  not  a  direct  effect  of  race,  is  proved  by 
the  fact  that  in  Europe,  the  rural  French  are  certainly  not 
more  prolific  than  the  rural  English. 


MULTIPLICATION    OF   THE    HUMAN    RACE.  483 

To  every  reader  there  will  probably  occur  the  seemingly 
adverse  evidence  furnished  by  the  Irish ;  who,  though  not 
well  fed,  multiply  fast.  Part  of  this  more  rapid  increase  is 
due  to  the  earlier  marriages  common  among  them,  and  con- 
sequent quicker  succession  of  generations — a  factor  which, 
as  we  have  seen,  has  a  larger  effect  than  any  other  on  the 
rate  of  multiplication.  Part  of  it  is  due  to  the  greater 
generality  of  marriage — to  the  comparative  smallness  of  the 
number  who  die  without  having  had  the  opportunity  of  pro- 
ducing offspring.  The  effects  of  these  causes  having  been 
deducted,  we  may  doubt  whether  the  Irish,  individually  con- 
sidered, would  be  found  more  prolific  than  the  English. 
Perhaps,  however,  it  will  be  said  that,  considering  their  diet, 
they  ought  to  be  less  prolific.  This  is  by  no  means  obvious. 
It  is  not  simply  a  question  of  nutriment  absorbed :  it  is  a 
question  of  how  much  remains  after  the  expenditure  in  self- 
maintenance.  Now  a  notorious  peculiarity  in  the  life  of  the 
Irish  peasant,  is,  that  he  obtains  a  return  of  food  that  is  large 
in  proportion  to  his  outlay  in  labour.  The  cultivation  of  his 
potatoe-ground  occupies  each  cottager  but  a  small  part  of  the 
year ;  and  the  domestic  economy  of  his  wife  is  not  of  a  kind 
to  entail  on  her  much  daily  exertion.  Consequently,  the  crop, 
tolerably  abundant  in  quantity  though  innutritive  in  quality, 
very  possibly  suffices  to  meet  the  comparatively- low  expendi- 
ture, and  to  leave  a  good  surplus  for  genesis — perhaps  a 
greater  surplus  than  remains  to  the  males  and  females  of  the 
English  peasantry,  who,  though  fed  on  better  food,  are 
harder  worked. 

We  conclude,  then,  that  in  the  human  race,  as  in  all  other 
races,  such  absolute  or  relative  abundance  of  nutriment  as 
leaves  a  large  excess  after  defraying  the  cost  of  carrying  on 
parental  life,  is  accompanied  by  a  high  rate  of  genesis.* 

*  This  is  exactly  the  reverse  of  Mr.  Doubleday's  doctrine ;  which  is  that 
throughout  both  the  animal  and  vegetal  kingdoms,  "  over-feeding  checks  in- 
crease ;  whilst,  on  the  other  hand,  a  limited  or  deficient  nutriment  stimu- 
lates aiid  adds  to  it."  Or,  as  he  elsewhere  says — "Be  the  range  of  the 


4  LA\VS   OF    Ml'LTlPl.ICATtOX. 

§  867.  Evidence  of  the  converse  truth,  that  relative  in 
crease  of  expenditure,  leaving  a  diminished  surplus,  reduces 
the  degree  of  fertility,  is  not  wanting.  Some  of  it  has  been 
set  down  for  the  sake  of  antithesis  in  the  foregoing  section. 
Here  may  be  grouped  a  few  facts  of  a  more  special  kind 
having  the  same  implication. 

To  prove  that  much  bodily  labour  renders  women  less 
prolific,  requires  more  evidence  than  is  obtainable.  Some  evi- 
dence, however,  may  be  set  down.  De  Boismont  in  France  and 
Dr.  Szukits  in  Austria,  have  shown  by  extensive  statistical 
comparisons,  that  the  reproductive  age  is  reached  a  year 
later  by  women  of  the  labouring  class  than  by  middle-class 
women ;  and  while  ascribing  this  delay  in  part  to  inferior 

natural  power  to  increase  in  any  species  what  it  may,  the  plethoric  state 
invariably  checks  it.  and  the  deplethoric  state  invariably  develops  it ;  and  this 
happens  in  the  exact  ratio  of  the  intensity  and  completeness  of  each  state, 
until  each  state  be  carried  so  far  as  to  bring  about  the  actual  death  of  the 
animal  or  plant  itself." 

I  have  space  here  only  to  indicate  the  misinterpretations  on  which  Mr. 
Doubleday  has  based  his  argument. 

In. the  firet  place,  he  has  confounded  normal  plethora  with  what  I  have,  in 
§  355,  distinguished  as  abnormal  plethora.  The  cases  of  infertility  accom- 
panying fatness,  which  he  cites  in  proof  that  over-feeding  checks  increase,  are 
not  cases  of  high  nutrition  properly  so  called  ;  but  cases  of  such  defective 
absorption  or  assimilation  as  constitutes  low  nutrition.  In  Chap.  IX,  abun- 
dant proof  was  given  that  a  truly  plethoric  state  is  an  unusually  fertile  state. 
It  may  be  added  that  much  of  the  evidence  by  which  Mr.  Doubleday  seeks  to 
ehow  that  among  men,  highly-fed  classes  are  infertile  classes,  may  be  out- 
balanced by  counter-evidence.  Many  years  ago  Mr.  Lewes  pointed  this  out : 
extracting  from  a  book  on  the  peerage,  the  names  of  16  peers  who  had,  at  that 
time,  186  children  ;  giving  an  average  of  11 '6  in  a  family. 

Mr.  Doubleday  insists  much  on  the  support  given  to  his  theory  by  the 
barrenness  of  very  luxuriant  plants,  and  the  fruitfulness  produced  in  plants 
by  depletion.  Had  he  been  aware  that  the  change  from  barrenness  to  fruit- 
fulness  in  plants,  is  a  change  from  agamogenesis  to  gamogenesis — had  it  been  as 
well  known  at  the  time  when  he  wrote  as  it  is  now,  that  a  tree  which  goes  on 
putting  out  sexless  shoots,  is  so  producing  new  individuals  ;  and  that  when  it 
begins  to  bear  fruit,  it  simply  begins  to  produce  new  individuals  after  another 
manner — he  would  have  perceived  that  facts  of  this  class  do  not  tell  in  his 
avour. 

In  the  law  which  Mr.  Doubleday  alleges,  he  sees  a  guarantee  for  the  main. 


MULTIPLICATION    OF   THE    HUMAN    RACE.  435 

nutrition,  we  may  suspect  that  it  is  in  part  duo  to  greater 
muscular  expenditure.  A  kindred  fact,  admitting  of  a 
kindred  interpretation,  m;iy  be  added.  Though  the  com- 
paratively-low rate  of  increase  in  France  is  attributed  to 
other  causes,  yet,  very  possibly,  one  of  its  causes  is  the  greater 
proportion  of  hard  work  entailed  on  French  women,  by  tho 
excessive  abstraction  of  men  for  non-productive  occupations, 
military  and  civil.  The  higher  rate  of  multiplication  in 
England  than  in  continental  countries  generall}',  is  not  im- 
probably furthered  by  the  easier  lives  which  English  women 
lend. 

That  absolute  or  relative  infertility  is  generally  pro- 
duced in  women  by  mental  labour  carried  to  excess,  is  more 
clearly  shown.  Though  the  regimen  of  upper-class  girls  is 
not  what  it  should  be,  yet,  considering  that  their  feeding  is 

tenance  of  species.  He  argnes  that  the  plethoric  state  of  the  individuals  con- 
stituting any  race  of  organisms,  presupposes  conditions  so  favourable  to  life 
that  the  race  can  be  in  no  danger ;  and  that  rapidity  of  multiplication  becomes 
needless.  Conversely,  he  argues  that  a  deplethoric  state  implies  unfavourable 
conditions— implies,  consequently,  unusual  mortality;  that  is— implies  a 
necessity  for  increased  fertility  to  prevent  the  race  from  dying  out.  It  mny 
be  readily  shown,  however,  that  such  an  arrangement  would  be  the  reverse  of 
self-adjnsting.  Suppose  a  species,  too  numerous  for  its  food,  to  be  in  the 
resulting  deplethoric  state.  It  will,  according  to  Mr.  Doubleday,  become 
unusually  fertile  ;  and  the  next  generation  will  be  more  numerous  rather  than 
less  numerous.  For,  by  the  hypothesis,  the  unusual  fertility  due  to  the 
deplethoric  state,  is  the  cause  of  undue  increase  of  population.  But  if  the 
next  generation  is  more  numerous  while  the  supply  of  food  has  remained 
the  same,  or  rather  has  decreased  under  the  keener  competition  for  it, 
then  this  next  generation  will  be  in  a  still  more  deplethoric  state,  and 
will  be  still  more  fertile.  Thus  there  will  go  on  an  ever-increasing  rate 
of  miltiplication,  and  an  ever-decreasing  supply  of  food,  until  the  species 
disappears.  Suppose,  ou  the  other  hand,  the  members  of  a  species  to  be  in 
an  unusually  plethoric  state.  Their  rate  of  multiplication,  ordinarily  suffi- 
cient to  maintain  their  numbers,  will  become  insufficient  to  maintain  their 
numbers.  In  the  next  generation,  therefore,  there  will  be  fewer  to  eat  the 
already  abundant  food,  which,  becoming  relatively  still  more  abundant,  will 
render  the  fewer  members  of  the  species  still  more  plethoric,  and  still  less 
fertile,  than  their  parents.  And  the  actions  and  reactions  continuing,  tht 
species  will  presently  die  out  frrfin  absolute  barrenness. 


LAWS   OF   MULTIPLICATION. 


better  than  that  of  girls  belonging  to  the  poorer  classesv 
while,  in  most  other  respects,  their  physical  treatment  is  not 
worse,  the  deficiency  of  reproductive  power  among  them 
may  be  reasonably  attributed  to  the  overtaxing  of  their 
brains  —  an  overtaxing  which  produces  a  serious  reaction  on 
the  physique.  This  diminution  of  reproductive  power  is  not 
shown  only  by  the  greater  frequency  of  absolute  sterility  ; 
nor  is  it  shown  only  in  the  earlier  cessation  of  child-bearing  ; 
but  it  is  also  shown  in  the  very  frequent  inability  of  such 
women  to  suckle  their  infants.  In  its  full  sense,  the  re- 
productive power  means  the  power  to  bear  a  well-  developed 
infant,  and  to  supply  that  infant  with  the  natural  food  for 
the  natural  period.  Most  of  the  flat-chested  girls  who 
survive  their  high-pressure  education,  are  incompetent  to 
do  this.  Were  their  fertility  measured  by  the  number  of 
children  they  could  rear  without  artificial  aid,  they  would 
prove  relatively  very  infertile. 

The  cost  of  reproduction  to  males  being  so  much  less 
than  it  is  to  females,  the  antagonism  between  Genesis  and 
Individuation  is  not  often  shown  in  men  by  suppression  of 
generative  power  consequent  on  unusual  expenditure  in 
bodily  action.  Nevertheless,  there  are  indications  that  this 
results  in  extreme  cases.  We  read  that  the  ancient  athletce 
rarely  had  children  ;  and  among  such  of  their  modern  repre- 
sentatives as  acrobats,  an  allied  relation  of  cause  and  effect 
is  alleged.  Indirectly  this  truth,  or  rather  its  converse, 
appears  to  have  been  ascertained  by  those  who  train  men  for 
feats  of  strength  —  they  find  it  needful  to  insist  on  con- 
tinence. 

Special  proofs  that  in  men,  great  cerebral  expenditure  di- 
minishes or  destroys  generative  power,  are  difficult  to  obtain. 
It  is,  indeed,  asserted  that  intense  application  to  mathematics, 
requiring  as  it  does  extreme  concentration  of  thought,  is  apt 
to  have  this  result  ;  and  it  is  asserted,  too,  that  this  result  is 
produced  by  the  excessive  emotional  excitement  of  gambling. 
Then,  again,  it  is  a  matter  of  common  remark  how  frequently 


MULTIPLICATION    OF   THE   HUMAN    RACE.  487 

men  of  unusual  mental  activity  leave  no  offspring.  But 
facts  of  this  kind  admit  of  another  interpretation.  The  re- 
action of  the  brain  on  the  body  is  so  violent — the  overtaxing 
of  the  nervous  system  is  so  apt  to  prostrate  the  heart  and 
derange  the  digestion  ;  that  the  incapacities  caused  in  these 
cases,  are  probably  often  due  more  to  constitutional  dis- 
turbance than  to  the  direct  deduction  which  excessive  action 
entails.  Such  instances  harmonize  with  the  hypothesis ;  but 
how  far  they  yield  it  positive  support  we  cannot  say. 

§  368.  An  objection  must  here  be  guarded  against.  It  ia 
likely  to  be  urged  that  since  the  civilized  races  are,  on  the 
average,  larger  than  many  of  the  uncivilized  races  ;  and  since 
they  are  also  somewhat  more  complex  as  well  as  more  active ; 
they  ought,  in  conformity  with  the  alleged  general  law,  to 
be  less  prolific.  There  is,  however,  no  evidence  to  prove  that 
they  are  so  :  on  the  whole,  they  seem  rather  the  reverse. 

The  reply  is  that  were  all  other  things  equal,  these 
superior  varieties  of  men  should  have  inferior  rates  of  in- 
crease. But  other  things  are  not  equal ;  and  it  is  to  the 
inequality  of  other  things  that  this  apparent  anomaly  is 
attributable.  Already  we  have  seen  how  much  more  fertile 
domesticated  animals  are  than  their  wild  kindred ;  and  the 
causes  of  this  greater  fertility  are  also  the  causes  of  the 
greater  fertility,  relative  or  absolute,  which  civilized  men 
exhibit  when  compared  with  savages. 

There  is  the  difference  in  amount  of  food.  Australians, 
Fuegians,  and  sundry  races  that  might  be  named  as  having 
low  rates  of  multiplication,  are  obviously  underfed.  The 
sketches  of  natives  contained  in  the  volumes  of  Livingstone, 
Baker,  and  others,  yield  clear  proofs  of  the  extreme 
depletion  common  among  the  uncivilized.  In 

quality  as  well  as  in  quantity,  their  feeding  is  bad.  Wild 
fruits,  insects,  larvoB,  vermin,  &c.,  which  we  refuse  with 
disgust,  often  enter  large!}7  into  their  dietary.  Much  of  this 
inferior  food  they  eat  uncooked ;  and  they  have  not  our 


188  LAWS   OF   MULTIPLICATION. 

elaborate  appliances  for  mechanically-preparing  it,  and 
rejecting  its  useless  parts.  So  that  they  live  on  matters  of 
less  nutritive  value,  which  cost  more  both  to  masticate  and 
to  digest.  Further,  to  uncivilized  men  supplies  of 

food  come  very  irregularly :  long  periods  of  scarcity  are 
divided  by  short  periods  of  abundance.  And  though  by 
gorging  when  opportunity  occurs,  something  is  done  towards 
compensating  for  previous  want,  yet  the  effects  of  prolonged 
starvation  cannot  be  neutralized  by  occasional  enormous 
meals.  Bearing  in  mind,  too,  that  improvident  as  they  are, 
savages  often  bestir  themselves  only  under  pressure  of 
hunger,  we  may  fairly  consider  them  as  habitually  ill- 
nourished — may  see  that  even  the  poorer  classes  of  civilized 
men,  making  regular  meals  on  food  separated  from  in- 
nutritive  matters,  easy  to  masticate  and  digest,  tolerably 
good  in  quality  and  adequate  if  not  abundant  in  quantity, 
are  much  better  nourished. 

Then,  again,  though  a  much  greater  consumption  in  mus- 
cular action  appears  to  be  undergone  by  civilized  men  than 
by  savages ;  and  though  it  is  probably  true  that  among  our 
labouring  people  the  daily  repairs  cost  more;  yet  in  many 
cases  there  does  not  exist  so  much  difference  as  we  are  apt  to 
suppose.  The  chase  is  very  laborious ;  and  great  amounts  of 
exertion  are  gone  through  by  the  lowest  races  in  seeking 
and  securing  the  odds  and  ends  of  wild  food  on  which 
they  largely  depend.  "We  naturally  assume  that  because 
barbarians  are  averse  to  regular  labour,  their  muscular 
action  is  less  than  our  own.  But  this  is  not  necessarily  true. 
The  monotonous  toil  is  what  the}T  cannot  tolerate  ;  and  they 
may  be  ready  to  go  through  as  much  or  more  exertion  when 
it  is  joined  with  excitement.  If  we  remember  that  the 
sportsman  who  gladly  scrambles  up  and  down  rough  hill- 
sides all  day  after  grouse  or  deer,  would  think  himself  hardly 
used  had  he  to  spend  as  much  effort  and  time  in  digging ;  we 
shall  see  that  a  savage  who  is  the  reverse  of  industrious, 
may  nevertheless  be  subject  to  a  muscular  waste  not  very 


MULTIPLICATION    OF   THE    HUMAN    RACE.  489 

different  in  amount  from  that  undergone  by  the  indus- 
trious. When  it  is  added  that  a  larger  physiolo- 
gical expenditure  is  entailed  on  the  uncivilized  than  on  the 
civilized  by  the  absence  of  good  appliances  for  shelter  and 
protection — that  in  some  cases  they  have  to  make  good  a 
greater  loss  of  heat,  and  in  other  cases  suffer  much  wear  from 
irritating  swarms  of  insects — we  shall  see  that  the  total  cost 
of  self-maintenance  among  them  is  probably  in  many  cases 
little  less,  and  in  some  cases  more,  than  it  is  among  ourselves. 
So  that  though,  on  the  average,  the  civilized  are  probably 
larger  than  the  savage ;  and  though  they  are,  in  their 
nervous  systems  at  least,  somewhat  more  complex ;  and 
though,  other  things  equal,  they  ought  to  be  the  less 
prolific ;  yet,  other  things  are  so  unequal,  as  to  make  it 
quite  conformable  to  the  general  law  that  they  should  be 
more  prolific.  In  §  365  we  observed  how,  among  inferior 
animals,  higher  evolution  sometimes  makes  self-preservation 
far  easier,  by  opening  the  way  to  resources  previously  un- 
available :  so  involving  an  undiminished,  or  even  an"  in- 
creased, rate  of  genesis.  And  similarly  we  may  expect 
among  races  of  men,  that  those  whose  slight  further  develop- 
ments have  been  followed  by  habits  and  arts  that  immensely 
facilitate  life,  will  not  exhibit  a  lower  degree  of  fertility,  and 
may  even  exhibit  a  higher. 

§  369.  One  more  objection  has  to  be  met — a  kindred  ob- 
jection to  which  there  is  a  kindred  reply.  Cases  may  be 
named  of  men  conspicuous  for  activity,  bodily  and  mental, 
Avho  were  also  noted,  not  for  less  generative  power  than  usual, 
but  for  more.  As  their  superiorities  indicate  higher  degrees 
of  evolution,  it  may  be  urged  that  such  men  should,  accord- 
ing to  the  theory,  have  lower  degrees  of  reproductive  activity. 
The  fact  that  here,  along  with  increased  powers  of  self-pre- 
servation, there  go  increased  powers  of  race-propagation, 
seems  irreconcilable  with  the  general  doctrine.  Reconcilia- 
tion is  not  difficult  however. 


490  LAWS   OF   MULTIPLICATION. 

The  cases  are  analogous  to  some  before  named,  in  whiyh 
more  abundant  food  simultaneously  aggrandizes  the  indi- 
vidual and  adds  to  the  production  of  new  individuals — the 
difference  between  the  cases  being,  that  instead  of  a  better 
external  supply  of  materials  there  is  here  a  better  internal 
utilization  of  materials.  Creatures  of  the  same  species  noto- 
riously differ  in  goodness  of  constitution.-  Here  there  is  some 
visceral  defect,  showing  itself  in  feebleness  of  all  the  func- 
tions; while  here  some  peculiarity  of  organic  balance,  some 
high  quality  of  tissue,  some  abundance  or  potency  of  the 
digestive  juices,  gives  to  the  system  a  perpetual  high  tide  of 
rich  blood,  that  serves  at  once  to  enhance  the  vital  activities 
and  to  raise  the  power  of  propagation.  Such  variations, 
however,  are  quite  independent  of  changes  in  the  proportion 
between  Individuation  and  Genesis  :  this  remains  the  same, 
while  both  are  increased  or  decreased  by  the  increase  or 
decrease  of  the  common  stock  of  materials. 

An  illustration  will  best  clear  up  any  perplexity.  Let  us 
say  that  the  fuel  burnt  in  the  furnace  of  a  locomotive  steam- 
engine,  answers  to  the  food  which  a  man  consumes ;  let  us 
say  that  the  produced  steam  expended  in  working  the  engine, 
corresponds  to  that  portion  of  absorbed  nutriment  which 
carries  on  the  man's  functions  and  activities ;  and  let  us 
say  that  the  steam  blowing  off  at  the  safety-valve, 
answers  to  that  portion  of  the  absorbed  nutriment  which 
goes  to  the  propagation  of  the  race.  Such  being  the  condi- 
tions of  the  case,  several  kinds  of  variations  are  possible. 
All  other  circumstances  remaining  the  same,  there  may  be 
changes  of  proportion  between  the  steam  used  for  working 
the  engine  and  the  steam  that  escapes  by  the  safety-valve. 
There  may  be  a  structural  or  organic  change  of  proportion. 
]$y  enlarging  the  safety-valve  or  weakening  its  spring,  while 
the  cylinders  are  reduced  in  size,  there  may  be  established  a 
constitutionally-small  power  of  locomotion  and  a  constitu- 
tionally-large amount  of  escape-steam  ;  and  inverse  variations 
so  produced,  will  answer  to  the  inverse  variations  between 


MULTIPLICATION   OF   THE    HUMAN    RACE.  491 

Individuation  and  Genesis  which  different  types  of  organisms 
show  us.  Again,  there  may  be  a  functional  change  of  pro- 
portion. If  the  engine  has  to  draw  a  considerable  load,  the 
abstraction  of  steam  by  the  cylinders  greatly  reduces  the 
discharge  by  the  safety-valve  ;  and  if  a  high  velocity  is  kept 
up,  the  discharge  from  the  safety-valve  entirely  ceases.  Con- 
versely, if  the  velocity  is  low,  the  escape-steam  bears  a  large 
ratio  to  the  steam  consumed  by  the  motor  apparatus ;  and  if 
the  engine  becomes  stationary  the  whole  of  the  steam  escapes 
by  the  safety-valve.  This  inverse  variation  answers  to  that 
which  we  have  traced  between  Expenditure  and  Genesis,  as 
displayed  in  the  contrasts  between  species  of  the  same  typo 
but  unlike  activities,  and  in  the  contrasts  between  active  and 
inactive  individuals  of  the  same  species.  But  now  beyond 
these  inverse  variations  between  the  quantities  of  consumed 
steam  and  escape-steam,  that  are  structurally  and  functionall}' 
caused,  there  are  coincident  variations,  producible  in  both  by 
changes  in  the  quantity  of  steam  supplied — changes  that 
may  be  caused  in  several  ways.  In  the  first  place,  the  fuel 
thrown  into  the  furnace  may  be  increased  or  made  better. 
Other  things  equal,  there  will  result  a  more  active  locomo- 
tion as  well  as  a  greater  escape ;  and  this  will  answer  to  that 
simultaneous  addition  to  its  individual  vigour  and  its  repro- 
ductive activity,  caused  in  an  animal  by  a  larger  quantity,  or 
a  superior  quality,  of  food.  In  the  second  place,  the  steam 
generated  may  be  economized.  Loss  by  radiation  from  the 
boiler  may  be  lessened  by  a  covering  of  non-conducting  sub- 
stances ;  and  part  of  the  steam  thus  prevented  from  con- 
densing, will  go  to  increase  the  working  power  of  the  engine, 
while  part  will  be  added  to  the  quantity  blowing  off.  This 
variation  corresponds  to  that  simultaneous  addition  to  bodily 
vigour  and  propagative  power,  which  results  in  animals  that 
have  to  expend  less  in  keeping  up  their  temperatures.  In 
the  third  place,  by  improvement  of  the  steam-generating 
apparatus,  more  steam  may  be  obtained  from  a  given  weight 
of  fuel.  A  better-formed  evaporating  surface,  or  boiler  plates 


192  LAWS   OF    MULTIPLICATION. 

which  conduct  more  rapidly,  or  an  increased  number  of  tubes, 
may  cause  a  larger  absorption  of  heat  from  the  burning  mass 
or  the  hot  gases  it  gives  off;  and  the  extra  steam  generated 
by  this  extra  heat,  will,  as  before,  augment  both  the  motive 
force  and  the  emission  through  the  safety-valve.  And  this 
last  case  of  coincident  variation,  is  parallel  to  the  case  with 
which  we  are  here  concerned — the  augmentation  of  indi- 
vidual expenditure  and  of  reproductive  energy,  that  may  be 
caused  by  a  superiority  of  some  organ  on  which  the  utilizing 
or  economizing  of  materials  depends. 

Manifestly,  therefore,  an  increased  expenditure  for  Genesis, 
or  an  increased  expenditure  for  Individuution,  may  arise  in 
one  of  two  quite  different  ways — either  by  diminution  of  the 
antagonistic  expenditure,  or  by  addition  to  the  store  which 
supplies  both  expenditures ;  and  confusion  results  from  not 
distinguishing  between  these.  Given  the  ratio  4  to  20,  as 
expressive  of  the  relative  costs  of  Genesis  and  Individuation, 
and  the  expenditure  for  Genesis  may  be  raised  to  5  while  the 
expenditure  for  Individuation  is  raised  to  25,  without  any 
alteration  of  tjTpe;  merely  by  favourable  circumstances  or 
superiority  of  constitution.  On  the  other  hand,  circumstances 
remaining  the  same,  the  expenditure  for  Genesis  "nay  be 
raised  from  4  to  5,  by  lowering  the  expenditure  for  Indi- 
viduation from  20  to  19 :  which  change  of  ratio  may  be 
either  functional  and  temporary,  or  structural  and  per- 
manent. And  only  when  it  is  the  last  does  it  illustrate  that 
inverse  variation  between  degree  of  evolution  and  degree  of 
procreative  dissolution,  which  we  have  everywhere  seen. 

§  370.  There  is  no  reason  to  suppose,  then,  that  the  laws 
of  multiplication  which  hold  of  other  beings,  do  not  hold  of 
the  human  being.  On  the  contrary,  there  are  special  facts 
which  unite  with  general  implications,  to  show  that  these 
laws  do  hold  of  the  human  being.  The  absence  of  direct 
evidence  in  some  cases  where  it  might  be  looked  for,  we  find 
fulty  explained  when  all  the  factors  are  taken  into  account. 


MULTIPLICATION    OF   THE   HUMAN    RACE.  493 

And  certain  seemingly-adverse  facts,  prove,  on  examination, 
to  be  facts  belonging  to  a  different  category  from  that  in 
which  they  are  placed,  and  harmonize  with  the  rest  when 
rightly  interpreted. 

The  conformity  of  human  fertility  to  the  laws  of  multipli- 
cation in  general,  being  granted,  it  remains  to  inquire  what 
effects  must  be  caused  by  permanent  changes  in  men's  natures 
and  circumstances.  Thus  far  we  have  observed  how,  by  their 
extremely-high  evolution  and  extremely-low  fertility,  man- 
kind display  the  inverse  variation  between  Individuation  and 
Genesis,  in  one  of  its  extremes.  And  we  have  also  observed 
how  mankind,  like  other  kinds,  are  functionally  changed  in 
their  rates  of  multiplication  by  changes  of  conditions.  But 
we  have  not  observed  how  alteration  of  structure  in  Man 
entails  alteration  of  fertility.  The  influence  of  this  factor  is 
BO  entangled  with  the  influences  of  other  factors  which  are 
for  the  present  more  important,  that  we  cannot  recognize  it. 
Here,  if  we  proceed  at  all,  we  must  proceed  deductively. 


CHAPTER  XIII. 

HUMAN  POPULATION  IN  THE  FUTURE. 

§  371.  Any  further  evolution  in  the  most-highly  evolved 
of  terrestrial  beings,  Man,  must  be  of  the  same  nature  as 
evolution  in  general.  Structurally  considered,  it  may  consist 
in  greater  integration,  or  greater  differentiation,  or  both — 
augmented  bulk,  or  increased  heterogeneity  and  definiteness, 
or  a  combination  of  the  two.  Functionally  considered,  it 
may  consist  in  a  larger  sum  of  actions,  or  more  multiplied 
varieties  of  actions,  or  both — a  larger  amount  of  sensible  and 
insensible  motion  generated,  or  motions  more  numerous  in 
kind  and  more  intricate  and  exact  in  co-ordination,  or 
motions  that  are  greater  alike  in  quantity,  complexity,  and 
precision. 

Expressing  the  change  in  terms  of  that  more  special 
evolution  displayed  by  organisms  ;  we  may  say  that  it  must 
be  one  which  further  adapts  the  moving  equilibrium  of 
organic  actions.  As  it  was  pointed  out  in  First  Principles, 
§  133,  "  the  maintenance  of  such  a  moving  equilibrium,  re- 
quires the  habitual  genesis  of  internal  forces  corresponding 
in  number,  directions,  and  amounts  to  the  external  incident 
forces — as  many  inner  functions,  single  or  combined,  as  there 
are  single  or  combined  outer  actions  to  be  met."  And  it 
was  also  pointed  out  that  "  the  structural  complexity  accom- 
panying functional  equilibration,  is  definable  as  one  in  which 
there  are  as  many  specialized  parts  as  are  cnpable,  separately 


HUMAN    POPULATION   IX   THE   FUTURE.  49-3 

and  jointly,  of  counteracting  the  separate  and  joint  forces 
amid  which  the  organism  exists."  Clearly,  then,  since  all 
incompletenesses  in  Man  as  now  constituted,  are  failures  to 
meet  certain  of  the  outer  actions,  mostly  involved,  remote, 
irregular,  to  which  he  is  exposed ;  every  advance  implies 
additional  co-ordinations  of  actions  and  accompanying  com- 
plexities of  organization. 

Or  once  more,  to  specialize  still  further  this  conception  of 
future  progress,  we  may  consider  it  as  an  advance  towards 
completion  of  that  continuous  adjustment  of  internal  to  ex- 
ternal relations,  which  constitutes  Life.  In  Part  I.  of  this 
work,  where  it  was  shown  that  the  correspondence  between 
inner  and  outer  actions  called  Life,  is  a  particular  kind  of 
what,  in  terms  of  Evolution,  we  called  a  moving  equilibrium  ; 
it  was  shown  that  the  degree  of  life  varies  as  the  degree  of 
correspondence.  Greater  evolution  or  higher  life,  implies, 
then,  such  modifications  of  human  nature  as  shall  make  more 
exact  the  existing  correspondences,  or  shall  establish  addi- 
tional correspondences,  or  both.  Connexions  of  phenomena 
of  a  rare,  distant,  unobtrusive,  or  intricate  kind,  which  we 
either  suffer  from  or  do  not  take  advantage  of,  have  to  be 
responded  to  by  new  connexions  of  ideas,  and  acts  properly 
combined  and  proportioned :  there  must  be  increase  of  know- 
ledge, or  skill,  or  power,  or  of  all  these.  And  to  effect  this 
more  extensive,  more  varied,  and  more  accurate,  co-ordina- 
tion of  actions,  there  must  be  organization  of  still  greater 
heterogeneity  and  definiteness. 

§  372.  Let  us  before  proceeding,  consider  in  what  par- 
ticular ways  this  further  evolution,  this  higher  life,  this 
greater  co-ordination  of  actions,  may  be  expected  to  show 
itself. 

Will  it  be  in  strength  ?  Probably  not  to  any  considerable 
degree.  Mechanical  appliances  are  fast  supplanting  brute 
force,  and  doubtless  will  continue  doing  this.  Though  at 
present  civilized  nations  largely  depend  for  self-preservation 


496  LAWS   OF   MULTIPLICATION. 

on  vigour  of  limb,  and  are  likely  to  do  so  while  wars  con- 
tinue ;  yet  that  progressive  adaptation  to  the  social  state  which 
must  at  last  bring  wars  to  an  end,  will  leave  the  amount  of 
muscular  power  to  adjust  itself  to  the  requirements  of  a 
peaceful  regime.  Though,  taking  all  things  into  account,  the 
muscular  power  then  required  may  not  be  less  than  now, 
there  seems  no  reason  why  more  should  be  required. 

Will  it  be  in  swiftness  or  agility  ?  Probably  not.  In  tho 
savages  these  are  important  elements  of  the  ability  to  main- 
tain life  ;  but  in  the  civilized  man  they  aid  self-preservation 
in  quite  a  minor  degree,  and  there  seems  no  circumstance 
likely  to  necessitate  an  increase  of  them.  By  games  and 
gymnastic  competitions,  such  attributes  may  indeed  be  arti- 
ficially increased;  but  no  artificial  increase  which  does  not 
bring  a  proportionate  advantage  can  be  permanent ;  since, 
other  things  equal,  individuals  and  societies  that  devote  the 
same  amounts  of  energy  in  ways  that  subserve  life  more 
effectually,  must  by  and  by  predominate. 

Will  it  be  in  mechanical  skill,  that  is,  in  the  better-co- 
ordination of  complex  movements?  Most  likely  in  some 
degree.  Awkwardness  is  continually  entailing  injuries  and 
deaths.  Moreover,  the  complicated  tools  which  civilization 
brings  into  use,  are  constantly  requiring  greater  delicacy  of 
manipulation.  All  the  arts,  industrial  and  aesthetic,  as  they 
develop,  imply  a  corresponding  development  of  perceptive  and 
executive  faculties  in  men — the  two  necessarily  act  and  react. 

Will  it  be  in  intelligence  ?  Largely,  no  doubt.  There  is 
ample  room  for  advance  in  this  direction,  and  ample  demand 
for  it.  Our  lives  are  universally  shortened  by  our  ignorance. 
In  attaining  complete  knowledge  of  our  own  natures  and  of 
the  natures  of  surrounding  things — in  ascertaining  the  con- 
ditions of  existence  to  which  we  must  conform,  and  in  dis- 
covering means  of  con  form  ing  to  them  under  all  variations 
of  seasons  and  circumstances — we  have  abundant  scope  for 
intellectual  progress. 

Will  it  be  in  morality,  that  is,  in  greater  power  of  self- 


HUMAN    POPULATION    IN   THE    FUTURE.  497 

regulation  ?  Largely  also :  perhaps  most  largely.  Bight 
conduct  is  usually  come  short  of  more  from  defect  of 
will  than  defect  of  knowledge.  To  the  due  co-ordination 
of  those  complex  actions  which  constitute  human  life  in 
its  civilized  form,  there  goes  not  only  the  pre-requisite 
— recognition  of  the  proper  course;  but  the  further  pre- 
requisite— a  due  impulse  to  pursue  that  course.  And  on 
calling  to  mind  our  daily  failures  to  fulfil  of  ten- repeated 
resolutions,  we  shall  perceive  that  lack  of  the  needful  desire, 
rather  than  lack  of  the  needful  insight,  is  the  chief  cause  of 
faulty  action.  A  further  endowment  of  those  feelings  which 
civilization  is  developing  in  us — sentiments  responding  to 
the  requirements  of  the  social  state — emotive  faculties  that 
find  their  gratifications  in  the  duties  devolving  on  us — must 
be  acquired  before  the  crimes,  excesses,  diseases,  improvi- 
dences, dishonesties,  and  cruelties,  that  now  so  greatly 
diminish  the  duration  of  life,  can  cease. 

Thus,  looking  at  the  several  possibilities,  and  asking 
what  direction  this  further  evolution,  this  more  complete 
moving  equilibrium,  this  better  adjustment  of  inner  to 
outer  relations,  this  more  perfect  co-ordination  of  actions, 
is  likely  to  take ;  we  conclude  that  it  must  take  mainly  the 
direction  of  a  higher  intellectual  and  emotional  develop- 
ment. 

§  373.  This  conclusion  we  shall  find  equally  forced  on  us 
if  we  inquire  for  the  causes  which  are  to  bring  about  such 
results.  No  more  in  the  case  of  Man  than  in  the  case  of  any 
other  being,  can  we  presume  that  evolution  either  has  taken 
place,  or  will  hereafter  take  place,  spontaneously.  In  the 
past,  at  present,  and  in  the  future,  all  modifications;  func- 
tional and  organic,  have  been,  are,  and  must  be  immediately 
or  remotely  consequent  on  surrounding  conditions.  What, 
then,  are  those  changes  in  the  environment  to  which,  by  direct 
or  indirect  equilibration,  the  human  organism  has  been 
adjusting  itself,  is  adjusting  itself  now,  and  will  continue  to 


198  LAWS   OF   MULTIPLICATION. 

adjust  itself?  And  how  do  they  necessitate  a  higher  evolu- 
tion of  the  organism  ? 

Civilization,  everywhere  having  for  its  antecedent  the  in- 
crease of  population,  and  everywhere  having  for  one  of  ita 
consequences  a  decrease  of  certain  race-destroying  forces,  has 
for  a  further  consequence  an  increase  of  certain  other  race- 
destroying  forces.  Danger  of  death  from  predatory  animals 
lessens  as  men  grow  more  numerous.  Though,  as  they  spread 
over  the  Earth  and  divide  into  tribes,  men  become  wild 
beasts  to  one  another,  yet  the  danger  of  death  from  this 
cause  also  diminishes  as  Iribes  coalesce  into  nations.  But  the 
danger  of  death  which  does  not  diminish,  is  that  produced  by 
augmentation  of  numbers  itself —the  danger  from  deficiency 
of  food.  Supposing  human  nature  to  remain  unchanged,  the 
mortality  hence  resulting  would,  on  the  average,  rise  aa 
human  beings  multiplied.  If  mortality,  under  such  condi- 
tions, does  not  rise,  it  must  be  because  the  supply  of  food 
also  augments ;  and  this  implies  some  change  in  human 
habits  wrought  by  the  stress  of  human  needs.  Here,  then,  is 
the  permanent  cause  of  modification  to  which  civilized  men 
are  exposed.  Though  the  intensity  of  its  action  is  ever  being 
mitigated  in  one  direction,  by  greater  production  of  food ;  it 
is,  in  the  other  direction,  ever  being  added  to  by  the  greater 
production  of  individuals.  Manifestly,  the  wants  of  their 
redundant  numbers  constitute  the  only  stimulus  mankind 
have  to  obtain  more  necessaries  of  life :  were  not  the  demand 
beyond  the  supply,  there  would  be  no  motive  to  increase  the 
supply.  And  manifestly,  this  excess  of  demand  over  supply 
is  perennial :  this  pressure  of  population,  of  which  it  is  the 
index,  cannot  be  eluded.  Though  by  the  emigration  that 
takes  place  when  the  pressure  arrives  at  a  certain  intensity, 
temporary  relief  is  from  time  to  time  obtained ;  yet  as,  by 
this  process,  all  habitable  countries  must  become  peopled,  it 
follows  that  in  the  end,  the  pressure,  whatever  it  may  then 
be,  must  be  borne  in  full. 

This  constant  increase  of  people  beyond  the  means  of  sub 


HUMAN   POrUJATION    IN   THE   FUTURE.  409 

sistence,  causes,  then,  a  never-ceasing  requirement  for  skill, 
intelligence,  and  self-control — involves,  therefore,  a  constant 
exercise  of  these  and  gradual  growth  of  them.  Every 
industrial  improvement  is  at  once  the  product  of  a  higher  form 
of  humanity,  and  demands  that  higher  form  of  humanity  to 
carry  it  into  practice.  The  application  of  science  to  the  arts, 
is  the  bringing  to  bear  greater  intelligence  for  satisfying  our 
wants ;  and  implies  continued  progress  of  that  intelligence. 
To  get  more  produce  from  the  acre,  the  farmer  must  study 
chemistry,  must  adopt  new  mechanical  appliances,  and  must, 
by  the  multiplication  of  processes,  cultivate  both  his  own 
powers  and  the  powers  of  his  labourers.  To  meet  the 
requirements  of  the  market,  the  manufacturer  is  per- 
petually improving  his  old  machines,  and  inventing  new 
ones  ;  and  by  the  premium  of  high  wages  incites  artizans  to 
acquire  greater  skill.  The  daily-widening  ramifications  of 
commerce  entail  on  the  merchant  a  need  for  more  know- 
ledge and  more  complex  calculations ;  while  the  lessening 
profits  of  the  ship-owner  force  him  to  build  more  scientifi- 
cally, to  get  captains  of  higher  intelligence,  and  better  crews. 
In  all  cases,  pressure  of  population  is  the  original  cause. 
Were  it  not  for  the  competition  this  entails,  more  thought 
and  energy  would  not  daily  be  spent  on  the  business  of  life ; 
and  growth  of  mental  power  would  not  take  place. 
Difficulty  in  getting  a  living  is  alike  the  incentive  to  a 
higher  education  of  children,  and  to  a  more  intense  and 
long- continued  application  in  adults.  In  the  mother  it  in- 
duces foresight,  economy,  and  skilful  house-keeping  ;  in  the 
father,  laborious  days  and  constant  self-denial.  Nothing  but 
necessity  could  make  men  submit  to  this  discipline;  and 
nothing  but  this  discipline  could  produce  a  continued  pro- 
gression. 

In  this  case,  as  in  many  others,  Nature  secures  each  step 
in  advance  by  a  succession  of  trials ;  which  are  perpetually 
repeated,  and  cannot  fail  to  be  repeated,  until  success  is 
achieved.  All  mankind  in  turn  subject  themselves  more  or 


500  LAWS   OF   MULTIPLICATION. 

less  to  the  discipline  described  ;  they  either  may  or  may  not 
advance  under  it ;  but,  in  the  nature  of  things,  only  those 
who  do  advance  under  it  eventually  survive.  For,  neces- 
sarily, families  and  races  whom  this  increasing  difficulty  of 
getting  a  living  which  excess  of  fertility  entails,  does  not 
stimulate  to  improvements  in  production — that  is,  to  greater 
mental  activity — are  on  the  high  road  to  extinction ;  and 
must  ultimately  be  supplanted  by  those  whom  the  pressure 
does  so  stimulate.  This  truth  we  have  recently  seen  exem- 
plified in  Ireland.  And  here,  indeed,  without  further 
illustration,  it  will  be  seen  that  premature  death,  under  all 
its  forms  and  from  all  its  causes,  cannot  fail  to  work  in  the 
same  direction.  For  as  those  prematurely  carried-off  mustj 
in  the  average  of  cases,  be  those  in  whom  the  power  of  self- 
preservation  is  the  least,  it  unavoidably  follows  that  those 
left  behind  to  continue  the  race,  must  be  those  in  whom 
the  power  of  self-preservation  is  the  greatest — must  be  the 
select  of  their  generation.  So  that,  whether  the  dangers  to 
existence  be  of  the  kind  produced  by  excess  of  fertility,  or  of 
any  other  kind,  it  is  clear  that  by  the  'ceaseless  exercise  of 
the  faculties  needed  to  contend  with  them,  and  by  the  death 
of  all  men  -who  fail  to  contend  with  them  successfully,  there 
is  ensured  a  constant  progress  towards  a  higher  degree  of 
skill,  intelligence,  and  self- regulation — a  better  co-ordina- 
tion of  actions — a  more  complete  life.* 

*  A  good  deal  of  this  chapter  retains  its  original  form ;  and  the  above 
paragraph  is  reprinted  verbatim  from  the  Westminster  Review  for  April,  1852, 
in  which  the  views  developed  in  the  foregoing  hundred  pages  were  first 
sketched  out.  This  paragraph  shows  how  near  one  may  be  to  a  great  generaliza- 
tion without  seeing  it.  Though  the  process  of  natural  selection  is  recognized-; 
and  though  to  it  is  ascribed  a  share  in  the  evolution  of  a  higher  type  ;  yet  the 
conception  must  not  be  confounded  with  that  which  Mr.  Darwin  has  worked 
out  with  such  wonderful  skill,  and  supported  by  such  vast  stores  of  knowledge. 
In  the  first  place,  natural  selection  is  here  described  only  as  furthering  direct 
adaptation — only  as  aiding  progress  by  the  preservation  of  individuals  in 
whom  functionally-produced  modifications  have  gone  on  most  favourably.  In 
the  second  place,  there  is  no  trace  of  the  idea  that  natural  selection  may,  by 
to-operation  with  the  cause  assigned,  or  with  other  causes,  produce  divergence* 


HUMAN   POPULATION    IN    THE   FUTURE.  501 

§  374.  The  proposition  at  which  we  have  thus  arrived,  is,, 
then,  that  excess  of  fertility,  through  the  changes  it  is  ever 
working  in  Man's  environment,  is  itself  the  cause  of  Man's 
further  evolution ;  and  the  obvious  corollary  here  to  be 
drawn,  is,  that  Man's  further  evolution  so  brought  about, 
itself  necessitates  a  decline  in  his  fertility. 

That  future  progress  of  civilization  which  the  never- 
ceasing  pressure  of  population  must  produce,  will  be  ac- 
companied by  an  enhanced  cost  of  Individuation,  both  in 
structure  and  function ;  and  more  especially  in  nervous 
structure  and  function.  The  peaceful  struggle  for  existence 
in  societies  ever  growing  more  crowded  and  more  complicated, 
must  have  for  its  concomitant  an  increase  of  the  great  nervous 
centres  in  mass,  in  complexity,  in  activity.  The  larger  body 
of  emotion  needed  as  a  fountain  of  energy  for  men  who  have 
to  hold  their  places  and  rear  their  families  under  the  inten- 
sifying competition  of  social  life,  is,  other  things  equal,  the 
correlative  of  larger  brain.  Those  higher  feelings  presupposed 
by  the  better  self-regulation  which,  in  a  better  society,  can 
alone  enable  the  individual  to  leave  a  persistent  posterity,  are, 
other  things  equal,  the  correlatives  of  a  more  complex  brain  ; 
as  are  also  those  more  numerous,  more  varied,  more  general, 
and  more  abstract  ideas,  which  must  also  become  increasingly 

of  structure ;  and  of  course,  in  the  absence  of  this  idea,  there  is  no  im- 
plication, even,  that  natural  selection  has  anything  to  do  with  the  origin  of 
species.  And  in  the  third  place,  the  all  important  factor  of  variation  — 
"spontaneous,"  or  incidental  as  AVC  may  otherwise  call  it — is  wholly  ignored. 
Though  use  and  disuse  are,  I  think,  much  more  potent  causes  of  organic 
modification  than  Mi.  Darwin  supposes — though,  while  pursuing  the  inquiry 
in  detail,  I  have  been  led  to  believe  that  direct  equilibration  has  played  a 
more  active  part  even  than  I  had  myself  at  one  time  thought  ;  yet  I  hold 
Mr.  Darwin  to  have  shown  beyond  question,  that  a  great  part  of  the  facts — 
perhaps  the  greater  part — are  explicable  only  as  resulting  from  the  survival  of 
individuals  which  have  deviated  in  some  indirectly-caused  way  from  the 
ancestral  type.  Thus,  the  above  paragraph  contains  merely  a  passing  recogni- 
tion of  the  selective  process ;  and  indicates  no  suspicion  of  the  enormous 
range  of  its  effects,  or  of  the  conditions  under  which  a  large  part  of  its  effects 
in;  produced. 


502  LAWS   OF   MULTIPLICATION. 

requisite  for  successful  life  as  society  advances.  And  the 
genesis  of  this  larger  quantity  of  feeling  and  thought,  in  a 
brain  thus  augmented  in  size  and  developed  in  structure,  is, 
other  things  equal,  the  correlative  of  a  greater  wear  of  nerv- 
ous tissue  and  greater  consumption  of  materials  to  repair  it. 
So  that  both  in  original  cost  of  construction  and  in  subse- 
quent cost  of  working,  the  nervous  system  must  become  a 
heavier  tax  on  the  organism.  Already  the  brain  of  the  civi- 
lized man  is  larger  by  nearly  thirty  per  cent,  than  the  brain 
of  the  savage.  Already,  too,  it  presents  an  increased  hetero- 
geneity-*— especially  in  the  distribution  of  its  convolutions. 
And  further  changes  like  these  which  have  taken  place 
under  the  discipline  of  civilized  life,  we  infer  will  continue 
to  take  place.  But  everywhere  and  always,  evolu- 

tion is  antagonistic  to  procreative  dissolution.  Whether  it 
be  in  greater  growth  of  the  organs  which  subserve  self- main- 
tenance, whether  it  be  in  their  added  complexity  of  structure, 
or  whether  it  be  in  their  higher  activity,  the  abstraction  of 
the  required  materials,  implies  a  diminished  reserve  of  ma- 
terials for  race-maintenance.  And  we  have  seen  reason  to 
believe  that  this  antagonism  between  Individuation  and 
Genesis,  becomes  unusually  marked  where  the  nervous  sys- 
tem is  concerned,  because  of  the  costliness  of  nervous  struc- 
ture and  function.  In  §  346  was  pointed  out  the  apparent 
connexion  between  high  cerebral  development  and  pro- 
longed delay  of  sexual  maturity ;  and  in  §  §  366,  367, 
the  evidence  went  to  show  that  where  exceptional  fer- 
tility exists  there  is  sluggishness  of  mind,  and  that  where 
there  has  been  during  education  excessive  expenditure  in 
mental  action,  there  frequently  follows  a  complete  or  partial 
infertility.  Hence  the  particular  kind  of  further  evolution 
which  Man  is  hereafter  to  undergo,  is  one  which,  more  than 
any  other,  may  be  expected  to  cause  a  decline  in  his  power  of 
reproduction. 

The  higher  nervous  development  and  greater  expenditure 
in  nervous  action,  here  described  as  indirectly  brought  about 


KUJAN    POPULATION    IX    THE    FUTURE.  503 

by  increase  of  numbers,  and  as  thereafter  becoming  a  check 
on.  the  increase  of  numbers,  must  not  be  taken  to  imply 
an  intenser  strain — a  mentally-laborious  life.  The  greater 
emotional  and  intellectual  power  and  activity  above  con- 
templated, must  be  understood  as  becoming,  by  small  incre- 
ments, organic,  spontaneous  and  pleasurable.  As,  even  when 
relieved  from  the  pressure  of  necessity,  large-brained  Euro- 
peans voluntarily  enter  on  enterprises  and  activities  which 
the  savage  could  not  keep  up  even  to  satisfy  urgent  wants ; 
so,  their  still  larger-brained  descendants  will,  in  a  still  higher 
degree,  find  their  gratifications  in  careers  entailing  still 
gi*eater  mental  expenditures.  This  enhanced  demand  for 
materials  to  establish  and  carry  on  the  psychical  functions, 
will  be  a  constitutional  demand.  "We  must  conceive  the 
type  gradually  so  modified,  that  the  more-developed  nervous 
system  irresistibly  draws  off,  for  its  normal  and  unforced 
activities,  a  larger  proportion  of  the  common  stock  of  nutri- 
ment ;  and  while  so  increasing  the  intensity,  completeness, 
and  length  of  the  individual  life,  necessarily  diminishing  the 
reserve  applicable  to  the  setting  up  of  new  lives — no  longer 
required  to  be  so  numerous. 

Though  the  working  of  this  process  will  doubtless  be 
interfered  with  and  modified  in  the  future,  as  it  has  been  in 
the  past,  by  the  facilitation  of  living  which  civilization 
brings ;  yet  nothing  beyond  temporary  interruptions  can  so 
be  caused.  However  much  the  industrial  arts  may  be  im- 
proved, there  must  be  a  limit  to  the  improvement;  while, 
with  a  rate  of  multiplication  in  excess  of  the  rate  of  mortality, 
population  must  continually  tread  on  the  heels  of  produc- 
tion. So  that  though,  during  the  earlier  stages  of  civiliza- 
tion, an  increased  amount  of  food  may  accrue  from  a  given 
amount  of  labour ;  there  must  come  a  time  when  this  relation 
will  be  reversed,  and  when  every  additional  increment  of 
food  will  be  obtained  by  a  more  than  proportionate  labour  : 
the  disproportion  growing  ever  higher,  and  the  diminution 
of  the  reproductive  power  becoming  greater. 
VOL.  II.  22 


504  LAWS   OF   MULTIPLICATION. 

§  375.  There  now  remains  but  to  inquire  towards  what 
limit  this  progress  tends.  So  long  as  the  fertility  of  the 
race  is  more  than  sufficient  to  balance  the  diminution  by 
deaths,  population  must  continue  to  increase.  So  long  as 
population  continues  to  increase,  there  must  be  pressure  on 
the  means  of  subsistence.  And  so  long  as  there  is  pressure 
on  the  means  of  subsistence,  further  mental  development  must 
go  on,  and  further  diminution  of  fertility  must  result.  Thus, 
the  change  can  never  cease  until  the  rate  of  multiplication  is 
just  equal  to  the  rate  of  mortality ;  that  is,  can  never  cease 
until,  on  the  average,  each  pair  has  as  many  children  as  are 
requisite  to  produce  another  generation  of  child-bearing 
adults,  equal  in  number  to  the  lust  generation.  At  first 
sight,  this  would  seem  to  imply  that  eventually  each  pair  will 
rarely  have  more  than  two  offspring  ;  but  a  little  considera- 
tion shows  that  this  is  a  lower  degree  of  fertility  than  is 
likely  ever  to  be  reached. 

Supposing  the  Sun's  light  and  heat,  on  which  all  terres- 
trial life  depends,  to  continue  abundant,  for  a  period  long 
enough  to  allow  the  entire  evolution  we  are  contemplating  ; 
there  are  still  certain  slow  astronomic  and  geologic  changes 
which  must  prevent  such  complete  adjustment  of  human  nature 
to  surrounding  conditions,  as  would  permit  the  rate  of  mul- 
tiplication to  fall  so  low.  As  before  pointed  out  (§  148) 
during  an  epoch  of  21,000  years,  each  hemisphere  goes 
through  a  cycle  of  temperate  seasons  and  seasons  extreme  in 
their  heat  and  cold  —  variations  that  are  themselves  alternately 
exaggerated  and  mitigated  in  the  course  of  far  longer  cycles  ; 
and  we  saw  that  these  caused  perpetual  ebbings  and 
flowings  of  species  over  different  parts  of  the  Earth's  surface. 
Further,  by  slow  but  inevitable  geologic  changes,  especially 
those  of  elevation  and  subsidence,  the  climate  and  physical 
characters  of  every  habitat  are  modified  ;  while  old  habitats 
are  destroyed  and  new  are  formed.  This,  too,  we  noted  as 
a  constant  cause  of  migrations  and  of  consequent  alterations 
of  environment.  Now  though  the  human  race  differs  from 


HUMAN    POPULATION    IN    THE    FUTURE.  505 

other  races  in  having  a  power  'f  artificially  counteracting 
external  changes,  yet  there  are  limits  to  this  power;  and, 
even  were  there  no  limits,  the  changes  could  not  fail  to 
work  their  effects  indirectly,  if  not  directly.  If,  as  is  thought 
probable,  these  astronomic  cycles  entail  recurrent  glacial  pe- 
riods in  each  hemisphere,  then,  parts  of  the  Earth  that  are  at 
one  time  thickly  peopled,  will  at  another  time,  be  almost  de- 
serted, and  vice  cersA.  The  geologically- caused  alterations  of 
climate  and  surface,  must  produce  further  slow  re-distributions 
of  population ;  and  other  currents  of  people,  to  and  from  different 
regions,  will  be  necessitated  by  the  rise  of  successive  centres 
of  higher  civilization.  Consequently,  mankind  cannot  but 
continue  to  undergo  changes  of  environment,  physical  and 
moral,  analogous  to  those  which  they  have  thus  far  been 
undergoing.  Such  changes  may  eventually  become  slower 
and  less  marked ;  but  they  can  never  cease.  And  if  they  can 
never  cease,  there  can  never  arise  a  perfect  adaptation  of 
human  nature  to  its  conditions  of  existence.  To  establish 
that  complete  correspondence  between  inner  and  outer  actions 
which  constitutes  the  highest  life  and  greatest  power  of  self- 
preservation,  there  must  be  a  prolonged  converse  between  the 
organism  and  circumstances  that  remain  the  same.  If  the 
external  relations  are  being  altered  while  the  internal  rela- 
tions are  being  adjusted  to  them,  the  adjustment  can  never 
become  exact.  And  in  the  absence  of  exact  adjustment, 
there  cannot  exist  that  theoretically- highest  power  of  self- 
preservation  with  which  there  would  co-exist  the  theoretically- 
lowest  power  of  race-production. 

Hence  though  the  number  of  premature  deaths  may  ul- 
timately become  very  small,  it  can  never  become  so  small 
as  to  allow  the  average  number  of  offspring  from  each  paii 
to  fall  so  low  as  two.  Some  average  number  between  two 
and  three  may  be  inferred  as  the  limit — a  number,  however, 
that  is  not  likely  to  be  quite  constant,  but  may  be  ex- 
pected at  one  time  to  increase  somewhat  and  afterwards 
to  decrease  somewhat,  according  as  variations  in  physical 


606  LAWS   OF    MULTIPLICATION. 

and  social  conditions  lower  or  raise  the  cost  of  self- 
preservation. 

Be  this  as  it  -may,  however,  it  is  manifest  that  in  the  end, 
pressure  of  population  and  its  accompanying  evils  will  dis- 
appear; and  will  leave  a  state  of  things  requiring  from  each 
individual  no  more  than  a  normal  and  pleasurable  activity. 
Cessation  in  the  decrease  of  fertility  implies  cessation  in 
the  development  of  the  nervous  system ;  and  this  implies  a 
nervous  system  that  has  become  equal  to  all  that  is  demanded 
of  it — has  not  to  do  more  than  is  natural  to  it.  But  that 
exercise  of  faculties  which  does  not  exceed  what  is  natural, 
constitutes  gratification.  In  the  end,  therefore,  the  ob- 
tainment  of  subsistence  and  discharge  of  all  the  parental 
arid  social  duties,  will  require  just  that  kind  and  that  amount 
of  action  needful  to  health  and  happiness. 

The  necessary  antagonism  of  Individuation  and  Genesis, 
not  only,  then,  fulfils  with  precision  the  a  priori  law  of 
maintenance  of  race,  from  the  Monad  up  to  Man,  but  ensures 
final  attainment  of  the  highest  form  of  this  maintenance  — 
a  form  in  which  the  amount  of  life  shall  be  the  greatest 
possible,  and  the  births  and  deaths  the  fewest  possible.  This 
antagonism  could  not  fail  to  work  out  the  results  we  see  it 
working  out.  The  excess  of  fertility  has  itself  rendered  the 
process  of  civilization  inevitable  ;  and  the  process  of  civiliza- 
tion must  inevitably  diminish  fertility,  and  at  last  destroy  its 
excess.  From  the  beginning,  pressure  of  population  has 
been  the  proximate  cause  of  progress.  It  produced  the 
original  diffusion  of  the  race.  It  compelled  men  to  abandon 
predatory  habits  and  take  to  agriculture.  It  led  to  the 
clearing  of  the  Earth's  surface.  It  forced  men  into  the 
social  state  ;  made  social  organization  inevitable  ;  and  has 
developed  the  social  sentiments.  It  has  stimulated  to  pro- 
gressive improvements  in  production,  and  to  increased  skill 
and  intelligence.  It  is  daily  thrusting  us  into  closer  contact 
and  more  mutually-dependent  relationships.  And  after  having 
caused,  as  it  ultimately  must,  the  due  peopling  of  the  globe, 


HUMAN    POPULATION    IN   THE    FUTURE.  507 

and  the  raising  of  all  its  habitable  parts  into  the  highest 
state  of  culture — after  having  brought  all  processes  for  the 
satisfaction  of  human  wants  to  perfection — after  having,  at 
the  same  time,  developed  the  intellect  into  complete  com- 
petency for  its  work,  and  the  feelings  into  complete  fitness 
for  social  life — after  having  done  all  this,  the  pressure  of 
population,  as  it  gradually  finishes  its  work,  must  gradually 
bring  itself  to  an  end. 

§  377.  In  closing  the  argument  let  us  not  overlook  the 
self-sufficingness  of  those  universal  processes  by  which  the 
results  reached  thus  far  have  been  wrought  out,  and  which 
may  be  expected  to  work  out  these  future  results. 

Evolution  under  all  its  aspects,  general  and  special,  is  an 
advance  towards  equilibrium.  We  have  seen  that  the  theo- 
retical limit  towards  which  the  integration  and  differentia- 
tion of  every  aggregate  advances,  is  a  state  of  balance  be- 
tween all  the  forces  to  which  its  parts  are  subject,  and 
the  forces  which  its  parts  oppose  to  them  (First  Prin.  §  130). 
And  we  have  seen  that  organic  evolution  is  a  progress 
towards  a  moving  equilibrium  completely  adjusted  to  en- 
vironing actions. 

It  has  been  also  pointed  out  that,  in  civilized  Man,  there  is 
going  on  a  new  class  of  equilibrations — those  between  his  ac- 
tions and  the  actions  of  the  societies  he  forms  (First  Prin. 
§  1 35).  Social  restraints  and  requirements  are  ever  altering 
his  activities  and  by  consequence  his  nature  ;  and  as  fast  as  his 
nature  is  altered,  social  restraints  and  requirements  undergo 
more  or  less  re- adjustment.  Here  the  organism  and  the  con- 
ditions are  both  modifiable ;  and  by  successive  conciliations 
of  the  two,  there  is  effected  a  progress  towards  equilibrium. 

More  recently  we  have  seen  that  in  every  species,  there 
establishes  itself  an  equilibrium  of  an  involved  kind  between 
the  total  race-destroying  forces  and  the  total  race- preserving 
forces — an  equilibrium  which  implies  that  where  the  ability 
to  maintain  individual  life  is  small,  the  ability  to  propagate 


503  I«AWS    OF    MULTIPLICATION. 

must  be  great,  and  vice  versd.  Whence  it  follows  that  the 
evolution  of  a  race  more  in  equilibrium  with  the  environment, 
is  also  the  evolution  of  a  race  in  which  there  is  a  correlative 
approach  towards  equilibrium  between  the  number  of  new 
individuals  produced  and  the  number  which  survive  and 
propagate. 

The  final  result  to  be  observed,  is,  that  in  Man,  all  these 
equilibrations  between  constitution  and  conditions,  between 
the  structure  of  society  and  the  nature  of  its  members,  be- 
tween fertility  and  mortality,  advance  simultaneously  towards 
a  common  climax.  In  approaching  an  equilibrium  between  his 
nature  and  the  ever-varying  circumstances  of  his  inorganic 
environment,  and  in  approaching  an  equilibrium  between  his 
nature  and  all  the  requirements  of  the  social  state,  Man  is  at 
the  same  time  approaching  that  lowest  limit  of  fertility  at 
which  the  equilibrium  of  population  is  maintained  by  the 
addition  of  as  many  infants  as  there  are  subtractions  bv  death 
in  old  age.  Changes  numerical,  social,  organic,  must,  by  their 
mutual  influences,  work  unceasingly  towards  a  state  of  har- 
mony— a  state  in  which  each  of  the  factors  is  just  equal  to  its 
work.  And  this  highest  conceivable  result  must  be  wrought 
out  by  that  same  universal  process  which  the  simplest  inor- 
ganic action  illustrates. 

THE   END. 


APPENDIX. 


APPENDIX    A. 


SUBSTITUTION  OF  AXIAL  FOR  FOLIAR  ORGANS  IN  PLANTS. 


I  APPEND  here  the  evidences  referred  to  in  §  190.  The  most 
numerous  and  striking  I  'have  met  with  among  the  Umbellifera. 
Monstrosities  having  the  alleged  implication,  are  frequent  in  the 
common  Cow-Parsnep — so  frequent  that  they  must  be  familiar  to 
botanists  ;  and  wild  Angelica  supplies  many  over-developments  of 
like  meaning.  Omitting  numerous  cases  of  more  or  less  significance, 
1  will  limit  myself  to  two. 

One  of  them  is  that  of  a  terminal  umbel,  in  which  nine  of  the  outer 
umbellules  are  variously  transformed — here  a  single  flower  being  made 
monstrous  by  the  development  of  some  of  its  members  into  buds  ; 
there  several  such  malformed  flowers  being  associated  with  rays  that 
bear  imperfect  umbellules  ;  and  elsewhere,  flowers  being  replaced  by 


ambellules  :  some  of  which  are  perfect,  and  others  imperfect  only  in 
the  shortness  of  the  flower-stalks.  The  annexed  Fig.  69,  represent- 
ing in  a  somewhat  conventionalized  way,  a  part  of  the  dried  speci» 


512 

men,  will  give  an  idea  of  this  Angelica.  At  a  is  shown  a  single 
flower  partially  changed ;  in  the  umbellule  marked  b,  one  of  the 
rays  bears  a  secondary  umbellule ;  and  there  may  be  seen  at  c  and 
d,  several  such  over-developments. 

But  the  most  conclusive  instance  is  that  of  a  Oow-Parsnep,  in  which 
a  single  terminal  umbel,  besides  the  transformations  already  men- 
tioned, exhibits  higher  degrees  of  such  transformations.*  The  com- 
ponents of  this  complex  growth  are ; — three  central  umbellules,  ab- 
normal only  in  minor  points  ;  one  umbellule,  external  to  these,  which 
is  partially  changed  into  an  umbel;  one  rather  more  out  of  the 
centre,  which  is  so  far  metamorphosed  as  to  be  more  an  umbel  than 
an  umbellule :  nine  peripheral  clusters  formed  by  the  development 
of  umbellules  into  umbels,  some  of  which  are  partially  compounded 
Btill  further.  Examined  in  detail,  these  structures  present  the  fol- 
lowing facts  : — 1.  The  innermost  umbellule  is  normal,  save  in  having 
a  peripheral  flower  of  which  one  member  (apparently  a  petal)  is 
transformed  into  a  flower-bud.  2.  The  next  umbellule,  not  quite  so 
central,  has  one  of  its  peripheral  flowers  made  monstrous  by  the 
growth  of  a  bud  from  the  base  of  the  calyx.  3.  The  third  of 
the  central  umbellules  has  two  abnormal  outer  flowers.  One  of 
them  carries  a  flower-bud  on  its  edge,  in  place  of  a  foliar  member. 
The  other  is  half  flower  and  half  umbellule  :  being  composed 
of  three  petals,  three  stamens,  and  five  flower-buds  growing 
where  the  other  petals  and  stamens  should  grow.  4.  Outside 
of  these  umbellules  comes  one  of  the  mixed  clusters.  Its  five 
central  flowers  are  normal.  Surrounding  these  are  several 
flowers  transformed  in  different  degrees  :  one  having  a  stamen  par- 
tially changed  into  a  flower  bud.  And  then,  at  the  periphery  of 
this  mixed  cluster,  come  three  complete  umbellules  and  an  incom- 
plete one  in  which  some  petals  and  stamens  of  the  original  flower 
remain.  5.  A  mixed  cluster,  in  which  the  umbel-structure  pre- 
dominates, stands  next.  Its  three  central  flowers  are  normal. 
Surrounding  them  are  five  flowers  over-developed  hi  various  ways, 
like  those  already  described.  And  on  its  periphery  are  seven 
complete  umbellules  in  place  of  flowers ;  besides  an  incomplete 
umbell ule  that  contains  traces  of  the  original  flower,  one  of  them 
being  a  petal  imperfectly  twisted  up  into  a  bud.  6.  Of  the  nine 
external  clusters,  in  which  the  development  of  simple  into  compound 
umbels  is  most  decided,  nearly  all  present  anomalies.  Three  of  them 
have  each  a  central  flower  uutransformed  ;  and  in  others,  the  central 

*  For  the  information  of  those  who  may  wish  to  examine  metamorphoses 
of  these  kinds,  I  may  here  state  that  I  have  found  nearly  all  the  examples 
described,  in  the  neighbourhood  of  the  sea — the  last-named,  on  the  shore  of 
Locheil,  near  Fort  William.  Whether  it  is  that  I  have  sought  more  dili- 
gently for  cases  when  in  such  localities,  or  whether  it  is  that  the  sea-air 
favours  that  excessive  nutrition  whence  these  transformations  result,  I  am 
unable  to  say. 


513 

ambellule  is  composed  of  two,  three,  or  four  flowers.  7.  But  the 
most  remarkable  fact  is,  that  in  sundry  of  these  peripheral  clusters, 
resulting  from  the  metamorphosis  of  simple  umbels  into  compound 
umbels,  the  like  metamorphosis  is  carried  a  stage  higher.  Some  of 
the  component  rays,  are  themselves  the  bearers  of  compound  umbels 


instead  of  simple  umbels.  In  Fig.  70,  a  portion  of  the  dried  speci- 
men is  represented.  Two  of  the  central  umbellules  are  marked  a 
and  b ;  those  marked  c  and  d  are  mixed  clusters ;  at  e  and  /  are 
compound  umbels  replacing  simple  ones ;  and  g  shows  one  of  the 
rays  on  which  the  over-development  goes  still  further. 

Does  not  this  evidence,  enforced  as  it  is  by  much  more  of  like 
kind,  go  far  to  prove  that  foliar  organs  may  be  developed  into  axial 
organs  ?  Even  were  not  the  transitional  forms  traceable,  there  would 
still,  I  think,  be  no  other  legitimate  interpretation  of  the  facts  last 
detailed.  The  only  way  of  eluding  the  conclusion  here  drawn,  is  by 
assuming  that  where  a  cluster  of  flowers  replaces  a  single  flower,  it 
is  because  the  axillary  buds  which  hypothetically  belong  to  the 
several  foliar  organs  of  the  flower,  become  developed  into  axes ;  and 
assuming  this,  is  basing  an  hypothesis  on  another  hypothesis  that  is 
directly  at  variance  with  facts.  The  foliar  organs  of  flowers  do  not 
bear  buds  in  their  axils ;  and  it  would  never  have  been  supposed 
that  such  buds  are  typically  present,  had  it  not  been  for  that 
mistaken  conception  of  "  type  "  which  has  led  to  many  other  errors 
in  Biology.  Goethe  writes:  "Now  as  we  cannot  realize  the  idea 
of  a  leaf  apart  from  the  node  out  of  which  it  springs,  or  of  a  node 
without  a  bud,  we  may  venture  to  infer,"  &c.  See  here  an  example 
of  a  method  of  philosophizing  not  uncommon  among  the  Germans. 


514 

The  method  is  this — Survey  a  portion  of  the  facts,  and  draw  from 
them  a  general  conception;  project  this  general  conception  back 
into  the  objective  world,  as  a  mould  in  which  Nature  casts  her 
products;  expect  to  find  it  everywhere  fulfilled;  and  allege  poten- 
tial fulfilment  where  no  actual  fulfilment  is  visible. 

If  instead  of  imposing  our  ideal  forms  on  Nature,  we  are  con- 
lent  to  generalize  the  facts  as  Nature  presents  them,  we  shall  find 
no  warrant  for  the  morphological  doctrine  above  enunciated.  The 
only  conception  of  type  justified  by  the  logic  of  science,  is — that 
correlation  of  parts  which  remains  constant  under  all  modifications 
of  the  structure  to  be  defined.  To  ascertain  this,  we  must  compare 
all  these  modifications,  and  note  what  traits  are  common  to  them. 
On  doing  so  with  the  successive  segments  of  a  phasnogamic  axis, 
we  are  brought  to  a  conclusion  widely  different  from  that  of  Goethe. 
Axillary  buds  are  almost  universally  absent  from  the  cotyledons ; 
they  are  habitually  present  in  the  axils  of  fully-developed  leaves 
higher  up  the  axis  ;  they  are  often  absent  from  leaves  that  are  close 
to  the  flower ;  they  are  nearly  always  absent  from  the  bracts  ;  absent 
from  the  sepals ;  absent  from  the  petals  ;  absent  from  the  stamens  ; 
absent  from  the  carpels.  Thus,  out  of  eight  leading  forms  which 
folia  assume,  one  has  the  axillary  bud  and  seven  are  without  it. 
With  these  facts  before  us,  it  seems  to  me  not  difficult  to  "  realize 
the  idea "  "  of  a  node  without  a  bud."  If  we  are  not  possessed 
by  a  foregone  conclusion,  the  evidence  will  lead  us  to  infer,  that 
each  node  bears  a  foliar  appendage  and  may  bear  an  axillary  bud. 

Even,  however,  were  it  granted  that  the  typical  segment  of  a 
Phasnogam  includes  an  axillary  bud,  which  must  be  regarded  as 
always  potentially  present,  no  legitimate  counter-interpretation  of 
the  monstrosities  above  described  could  thence  be  drawn.  If  when 
an  umbellule  is  developed  in  place  of  a  flower,  the  explanation  is, 
that  its  component  rays  are  axillary  to  the  foliar  organs  of  the 
flower  superseded  ;  we  may  fairly  require  that  these  foliar  organs  to 
which  they  are  axillary,  shall  be  shown.  But  there  are  none.  In 
the  last  specimen  figured,  the  inner  rays  of  each  such  umbellule  are 
without  them ;  most  of  the  outer  rays  are  also  without  them ;  and 
in  one  cluster,  only  a  single  ray  has  a  bract  at  its  point  of  origin. 
There  is  a  rejoinder  ready,  however :  the  foliar  organs  are  said  to 
be  suppressed.  Though  Goethe  could  not  "  realize  the  idea"  "  of 
n  node  without  a  bud,"  those  who  accept  his  typical  form  appear  to 
find  no  difficulty  in  realizing  the  idea  of  an  axillary  bud  without 
anything  to  which  it  is  axillary.  But  letting  this  pass,  suppose  we 
ask  what  is  the  warrant  for  this  assumed  suppression.  Axillary 
buds  normally  occur  where  the  nutrition  is  high  enough  to  produce 
fully-developed  leaves;  and  when  axillary  buds  are  demonstrably 
present  in  flowers,  they  accompany  foliar  organs  that  are  more  leaf- 
like  than  usual — always  greener  if  not  always  larger.  That  is  to 


515 

say,  the  normal  and  the  abnormal  axillary  buds,  are  alike  the  con- 
comitants of  foliar  organs  coloured  by  that  chlorophyll  which 
habitually  favours  foliar  development.  How,  then,  can  it  be  sup- 
posed that  when,  out  of  a  flower  there  is  developed  a  cluster  of 
flower-bearing  rays,  -the  implied  excess  of  nutrition  causes  the  foliar 
organs  to  abort  ?  It  is  true  that  very  generally  in  a  branched  in- 
florescence, the  bracts  of  the  several  flower-branches  are  very  small 
(their  smallness  being  probably  due  to  that  defective  supply  of 
certain  chlorophyll-forming  matters,  which  is  the  proximate  cause 
of  flowering)  ;  and  it  is  true  that,  under  these  conditions,  a  flower- 
ing axis  of  considerable  size,  for  the  development  of  which  chloro- 
phyll is  less  needful,  grows  from  the  axil  of  a  dwarfed  leaf.  But 
the  inference  that  the  foliar  organ  may  therefore  be  entirely  sup- 
pressed, seems  to  me  irreconcilable  with  the-  fact,  that  the  foliar 
organ  is  always  developed  to  some  extent  before  the  axillary  bud 
appears.  Until  it  has  been  shown  that  in  some  cases  a  lateral  bud 
first  appears,  and  a  foliar  organ  afterwards  grows  out  beneath  it,  to 
form  its  axil,  the  conception  of  an  axillary  bud  of  which  the'  foliar 
organ  is  suppressed,  will  remain  at  variance  with  the  established 
truths  of  development. 


The  above  originally  formed  a  portion  of  §  190.  I  have  transferred 
it  to  the  Appendix,  partly  because  it  contains  too  much  detail  to 
render  it  fit  for  the  general  argument,  and  partly  because  the  inter- 
pretations being  open  to  some  question,  it  seemed  undesirable  to  risk 
compromising  that  argument  by  including  them.  The  criticisms 
passed  upon  these  interpretations  have  not,  however,  sufficed  to  con- 
vince me  of  their  incorrectness.  Unfortunately,  I  have  since  had  no 
opportunity  of  verifying  the  above  statements  by  microscopic  exami- 
nations, as  I  had  intended. 

Though  unable  to  enforce  the  inference  drawn  by  further  facts 
more  minutely  looked  into,  I  may  add  some  arguments  based  on 
facts  that  are  well  known.  One  of  these  is  the  fact  that  the  so- 
called  axillary  bud  is  not  universally  axillary — is  not  universally 
seated  in  the  angle  made  by  the  axis  and  an  appended  foliar  organ. 
In  certain  plants  the  axillary  bud  is  placed  far  above  the  node, 
half-way  between  it  and  the  succeeding  node.  So  that  not  only  may 
a  segment  of  a  phamogamic  axis  be  without  the  axillary  bud,  but 
the  axillary  bud,  when  present,  may  be  removed  from  that  place  in 
which,  according  to  Goethe,  it  necessarily  exists.  Another  fact  not 
congruous  with  the  current  doctrine,-  is  the  common  occurrence  of 
"adventitious"  buds — the  buds  that  are  put  out  from  roots  and  from 
old  stems  or  branches  bare  of  leaves.  The  name  under  which  they  are 
thus  classed,  is  meant  to  imply  that  they  may  be  left  out  of  conside- 
ration. Those,  however,  who  have  not  got  a  theory  to  save  by 


616 

putting  anomalies  out  of  sight,  may  be  inclined  to  think  that  the 
occurrence  of  buds  where  they  are  avowedly  unconnected  with 
nodes,  and  are  axillary  to  nothing,  tells  very  much  against  the  as- 
sumption that  every  bud  implies  a  node  and  a  corresponding  foliar 
organ.  And  they  may  also  see  that  the  development  of  these  ad- 
ventitious buds  at  places  where  there  is  excess  of  nutritive  mate- 
rials, favours  the  view  above  set  forth.  For  if  a  bud  thus  arises  at 
a  place  where  it^s  not  morphologically  accounted  for,  simply  because 
there  happens  to  be  at  that  place  an  abundance  of  unorganized  pro- 
toplasm ;  then,  clearly,  it  is  likely  that  if  the  mass  of  protaplasm 
from  which  a  leaf  would  usually  arise,  is  greatly  increased  hi  mass 
bv  excess  of  nutrition,  it  may  develop  into  an  axis  instead  of  a  leaf 


APPENDIX    B. 


A   CRITICISM   ON   PROF.    OWEN'S   THEORY   OF   THE 
VERTEBRATE  SKELETON. 


[From  the  BRITISH  &  FOREIGN  MEDICO-CHIRURGICAL  REVIEW  FOR  OCT.,  1858.] 


I.  On  the  Archetype  and  Homologies  of  the  Vertebrate  Skeleton.     By 
RICHARD  OWEN,  F.R.S. — London,  1848.    pp.  172. 

II.  Principes  tfOsteologie  Comparee,  ou  Recherches  sur  f Archetype 
et  les  Homologies  da  Squelette  Vertebre.     Par  RICHARD  OWEN. — 
Paris. 

Principles  oj  Comparative  Osteology  ;  or,  Researches  on  the  Arcnetype 
and  the  Homologies  oj  the  Vertebrate  Skeleton.  By  RICHARD 
OWEN. 

III.  On  the  Nature  of  Limbs.     A  Discourse  delivered  on  Friday, 
February  9,  at  an  Evening  Meeting  of  the  Royal  Institution  of 
Great  Britain.     By   RICHARD   OWEN,  F.R.S. — London,    1849. 

pp.  119. 

JUDGING  whether  another  proves  his  position  is  a  widely  different 
thing  from  proving  your  own.  To  establish  a  general  law  requires 
an  extensive  knowledge  of  the  phenomena  to  be  generalized ;  but  to 
decide  whether  an  alleged  general  law  is  established  by  the  evidence 
assigned,  requires  merely  an  adequate  reasoning  faculty.  Especially 
is  such  a  decision  easy  where  the  premises  do  not  warrant  the  con- 
clusion, Jt  may  be  dangerous  for  one  who  has  but  little  previous 
acquaintance  with  the  facts,  to  say  that  a  generalization  is  demon- 
strated ;  seeing  that  the  argument  may  be  one-sided :  there  may  be 
many  facts  unknown  to  him  which  disprove  it.  But  it  is  not 
dangerous  to  give  a  negative  verdict  when  the  alleged  demonstra- 


518 

tioii  is  manifestly  insufficient.  If  the  data  put  before  him  do  not 
bear  out  the  inference,  it  is  competent  for  every  logical  reader  to 
say  so. 

From  this  stand-point,  then,  we  venture  to  criticize  some  of 
Professor  Owen's  osteological  theories.  For  his  knowledge  of 
comparative  osteology  we  have  the  highest  respect.  Wo  believe 
that  no  living  man  has  so  wide  and  detailed  an  acquaintance  with 
the  bony  structure  of  the  Vertebrata.  Indeed,  there  probably  has 
Dover  been  any  one  whose  information  on  the  subject  was  so  nearly 
exhaustive.  Moreover,  we  confess  that  nearly  all  we  know  of  this 
department  of  biology  has  been  learnt  from  his  lectures  and  writ- 
ings. We  pretend  to  no  independent  investigations,  but  merely  to 
such  knowledge  of  the  phenomena  as  he  has  furnished  us  with. 
Our  position,  then,  is  such  that,  had  Professor  Owen  simply  enun- 
ciated his  generalizations,  we  should  have  accepted  them  on  his 
authority.  But  he  has  brought  forward  evidence  to  prove  them. 
By  so  doing  he  has  tacitly  appealed  to  the  judgments  of  his  readers 
and  hearers — has  practically  said,  "  Here  are  the  facts ;  do  they 
not  warrant  these  conclusions  f '  And  all  we  propose  to  do,  is  to 
consider  whether  the  conclusions  are  warranted  by  the  facts  brought 
forward. 

Let  us  first  limit  the  scope  of  our  criticisms.  On  that  division 
of  comparative  osteology  which  deals  with  what  Professor  Owen 
distinguishes  as  "  special  homologies,"  we  do  not  propose  to  enter. 
That  the  wing  of  a  bird  is  framed  upon  bones  essentially  parallel  to 
those  of  a  mammal's  fore-limb ;  that  the  cannon-bone  of  a  horse's 
leg  answers  to  the  middle  metacarpal  of  the  human  hand  ;  that 
various  bones  in  the  skull  of  a  fish  are  homologous  with  bones  in 
the  skull  of  a  man — these  and  countless  similar  facts,  we  take  to  be 
well  established.  It  may  be,  indeed,  that  the  doctrine  of  special 
homologies  is  at  present  carried  too  far.  It  may  be  that,  just  as 
the  sweeping  generalization  at  one  time  favoured,  that  the  embryonic 
phases  of  the  higher  animals  represent  the  adult  forms  of  lower 
ones,  has  been  found  untrue  in  a  literal  sense,  and  is  acceptable 
only  in  a  qualified  sense ;  so  the  sweeping  generalization  that  the 
skeletons  of  all  vertebrate  animals  consist  of  homologous  parts,  will 
have  to  undergo  some  modification.  But  that  this  generalization 
is  substantially  true,  all  comparative  anatomists  agree. 

The  doctrine  which  we  are  here  to  consider,  is  quite  a  separate 
one — that  of  "  general  homologies."  The  truth  or  falsity  of  this 
may  be  decided  on  quite  apart  from  that  of  the  other.  Whether 
certain  bones  in  one  vertebrate  animal's  skeleton  correspond  with 
certain  bones  in  another's,  or  in  every  other's,  is  one  question ;  and 
whether  the  skeleton  of  every  vertebrate  animal  is  divisible  into  a 
series  of  segments,  each  of  which  is  modelled  after  the  same  type, 
is  another  question.  While  the  first  is  answered  in  the  affirmative, 


519 

the  last  may  be  answered  in  the  negative ;  and  we  propose  to  give 
reasons  why  it  should  be  answered  in  the  negative. 

In  so  far  as  his  theory  of  the  skeleton  is  concerned,  Professor 
Owen  is  an  avowed  disciple  of  Plato.  At  the  conclusion  of  his 
A  rclietype  and  limnologies  of  the  Vertebrate  Skeleton,  he  quotes  ap- 
provingly the  Platonic  hypothesis  of  $exi,  "  a  sort  of  models,  or 
moulds  in  which  matter  is  cast,  and  which  regularly  produce  the 
same  number  and  diversity  of  species."  The  vertebrate  form  in 
general  (see  diagram  of  the  Archetypus),  or  else  the  form  of  each 
kind  of  vertebrate  animal  (see  p.  172,  where  this  seems  implied), 
Professor  Owen  conceives  to  exist  as  an  "  idea " — an  "  arche- 
typal exemplar  on  which  it  has  pleased  the  Creator  to  frame 
certain  of  his  living  creatures."  Whether  Professor  Owen  holds 
that  the  typical  vertebra  also  exists  as  an  "  idea,"  is  not  so 
certain.  From  the  title  given  to  his  figure  of  the  "  ideal  typical 
vertebra,"  it  would  seem  that  he  does;  and  at  p.  40  of  his 
Mature  of  Limbs,  and  indeed  throughout  his  general  argument,  this 
supposition  is  implied.  But  on  the  last  two  pages  of  the  Archetype 
and  Jlomologies,  it  is  distinctly  alleged  that  "  the  repetition  of  simi- 
lar segments  in  a  vertebral  column,  and  of  similar  elements  in  a 
vertebral  segment,  is  analogous  to  the  repetition  of  similar  crystals 
as  the  result  of  polarizing  force  in  the  growth  of  an  inorganic 
body ;  "  it  is  pointed  out  that,  "  as  we  descend  the  scale  of  animal 
life,  the  forms  of  the  repeated  parts  of  the  skeleton  approach  more 
and  more  to  geometrical  figures ; "  and  it  is  inferred  that  "  the 
Platonic  $e'«  or  specific  organizing  principle  or  force,  would  seem 
to  be  in  antagonism  with  the  general  polarizing  force,  and  to  sub- 
due and  mould  it  in  subserviency  to  the  exigencies  of  the  resulting 
specific  form."  If  Professor  Owen's  doctrine  is  to  be  understood 
as  expressed  in  these  closing  paragraphs  of  his  Archetype  and  Homo- 
loc/ies — if  he  considers  that  "  the  lUa  "  "  which  produces  the  diver- 
sity of  form  belonging  to  living  bodies  of  the  same  materials,"  is 
met  by  the  "  counter-operation  "  of  "  the  polarizing  force  pervading 
all  space,"  which  produces  "  the  similarity  of  forms,  the  repetition 
of  parts,  the  signs  of  unity  of  organization,"  and  which  is  "  subdued  " 
as  we  ascend  "  in  the  scale  of  being ;  "  then  we  may  pass  on  with 
the  remark  that  the  hypothesis  is  too  cumbrous  and  involved  to 
have  much  vraisemblance.  If,  on  the  other  hand,  Professor  Owen 
holds,  as  every  reader  would  suppose  from  the  general  tenor  of  his 
reasonings,  that  not  only  does  there  exist  an  archetypal  or  ideal 
vertebrate  skeleton,  but  that  there  also  exists  an  archetypal  or 
ideal  vertebra;  then  he  carries  the  Platonic  hypothesis  much 
further  than  Plato  does.  Plato's  argument,  that  before  any  species 
of  object  was  created  it  must  have  existed  as  an  idea  of  the  Creative 
Intelligence,  and  that  hence  all  objects  of  such  species  must  be 


520 

copies  of  this  original  idea,  is  tenable  enough  from  the  anthropo- 
morphic point  of  view.  But  while  those  who,  with  Plato,  think  fit 
to  base  their  theory  of  creation  upon  the  analogy  of  a  carpenter 
designing  and  making  a  table,  must  yield  assent  to  Plato's  inference, 
they  are  by  no  means  committed  to  Professor  Owen's  expansion  of 
it.  To  say  that  before  creating  a  vertebrate  animal,  God  must 
have  had  the  conception  of  one,  does  not  involve  saying  that  God 
gratuitously  bound  himself  to  make  a  vertebrate  animal  out  of  seg- 
ments all  moulded  after  one  pattern.  As  tnere  is  no  conceivable 
advantage  in  this  alleged  adhesion  to  a  fundamental  pattern — as, 
for  the  fulfilment  of  the  intended  ends,  it  is  not  only  needless,  but 
often,  as  Professor  Owen  argues,  less  appropriate  than  some  other 
construction  would  be  (see  Nature  of  Limbs,  pp.  39,  40),  to  sup- 
pose the  creative  processes  thus  regulated,  is  not  a  little  startling. 
Even  those  whose  conceptions  are  so  anthropomorphic  as  to  think 
they  honour  the  Creator  by  calling  him  "  the  Great  Artificer,"  will 
scarcely  ascribe  to  him  a  proceeding  which,  in  a  human  artificer, 
they  would  consider  a  not  very  worthy  exercise  of  ingenuity. 

But  whichever  of  these  alternatives  Professor  Owen  contends  for 
—whether  the  typical  vertebra  is  that  more  or  less  crystalline  figure 
which  osseous  matter  ever  tends  to  assume  in  spite  of  "  the  *3ea  or 
organizing  principle,"  or  whether  the  typical  vertebra  is  itself  an 
"  tie*  or  organizing  principle" — there  is  alike  implied  the  belief 
that  the  typical  vertebra  has  an  abstract  existence  apart  from  actual 
vertebrae.  It  is  a  form  which,  in  every  endoskeleton,  strives  to 
embody  itself  in  matter — a  form  which  is  potentially  present  in  each 
vertebra ;  which  is  manifested  in  each  vertebra  with  more  or  less 
clearness  ;  but  which,  in  consequence  of  antagonizing  forces,  is  no- 
where completely  realized.  Apart  from  the  philosophy  of  this 
hypothesis,  let  us  here  examine  the  evidence  which  is  thought  to 
justify  it. 

And  first  as  to  the  essential  constituents  of  the  "  ideal  typical 
vertebra."  Exclusive  of  "  diverging  appendages  "  which  it  "  may 
also  support,"  "  it  consists  in  its  typical  completeness  of  the  follow- 
ing elements  and  parts": — A  centrum  round  which  the  rest  are 
arranged  in  a  somewhat  radiate  manner ;  above  it  two  neurapophyses 
— converging  as  they  ascend,  and  forming  with  the  centrum  a  trian- 
guloid  space  containing  the  neural  axis ;  a  neural  spine  surmounting 
the  two  neurapophyses,  and  with  them  completing  the  neural  arch  ; 
below  the  centrum  two  hamapophyses  and  a  hcemal  spine,  forming  a 
haemal  arch  similar  to  the  neural  arch  above,  and  enclosing  the 
haemal  axis ;  two  pleurapophyses  radiating  horizontally  from  the 
sides  of  the  centrum ;  and  two  parapophyses  diverging  from  the 
Centrum  below  the  pleurapophyses.  "  These,"  says  Professor 
Owen,  "  being  usually  developed  from  distinct  and  independent 


521 

centres,  I  have  termed  *  autogenous  elements.' "  The  remaining 
elements,  which  he  classes  as  "  exogenous,"  because  they  "  shoot 
out  as  continuations  from  some  of  the  preceding  elements,"  are  the 
diapophyses  diverging  from  the  upper  part  of  the  centrum  as  the 
parapophyses  do  below,  and  the  zygapophyses  which  grow  out  of  the 
distal  ends  of  the  neurapophyses  and  hasmapophyses. 

If,  now,  these  are  the  constituents  of  the  vertebrate  segment  "  in 
its  typical  completeness  ;"  and  if  the  vertebrate  skeleton  consists  of 
a  succession  of  such  segments  ;  we  ought  to  have  in  these  con- 
stituents, representatives  of  all  the  elements  of  the  vertebrate 
skeleton — at  any  rate,  all  its  essential  elements.  Are  we  then  to 
conclude  that  the  "  diverging  appendages,"  which  Professor  Owen 
regards  as  rudimental  limbs,  and  from  certain  of  which  he  considers 
actual  limbs  to  be  developed,  are  typically  less  important  than  some 
of  the  above-specified  exogenous  parts — say  the  zygapophyses  ? 

That  the  meaning  of  this  question  may  be  understood,  it  will  be 
needful  briefly  to  state  Professor  Owen's  theory  of  The  Nature  of 
Limbs ;  and  such  criticisms  as  we  have  to  make  on  it  must  be  in- 
cluded in  the  parenthesis.  In  the  first  place,  he  aims  to  show  that 
the  scapular  and  pelvic  arches,  giving  insertion  to  the  fore  and  hind 
limbs  respectively,  are  displaced  and  modified  hasmal  arches, 
originally  belonging  in  the  one  case  to  the  occipital  vertebra,  and  in 
the  other  case  to  some  trunk-vertebra  not  specified.  In  support  of 
this  assumption  of  displacement,  carried  in  some  cases  to  the  extent 
of  twenty-seven  vertebrae,  Professor  Owen  cites  certain  acknow- 
ledged displacements  which  occur  in  the  human  skeleton  to  the  ex- 
tent of  half  a  vertebra — a  somewhat  slender  justification. .  But  for 
proof  that  such  a  displacement  has  taken  place  in  the  scapular  arch, 
he  chiefly  relies  on  the  fact  that  hi  fishes,  the  pectoral  fins,  which 
are  the  homologues  of  the  fore-limbs,  are  directly  articulated  to 
certain  bones  at  the  back  of  the  head,  which  he  alleges  are  parts 
of  the  occipital  vertebra.  This  appeal  to  the  class  of  fishes  is 
avowedly  made  on  the  principle  that  these  lowest  of  the  Vertebrata 
approach  closest  to  archetypal  regularity,  and  may  therefore  be 
expected  to  show  the  original  relations  of  the  bones  more  nearly. 
Simply  noting  the  facts  that  Professor  Owen  does  not  give  us  any 
transitional  forms  between  the  alleged  normal  position  of  the 
scapular  arch  in  fishes,  and  its  extraordinary  displacement  in  tho 
higher  Vertebrata ;  and  that  he  makes  no  reference  to  the  embryonic 
phases  of  the  higher  Vertebrata,  which  might  be  expected  to  ex- 
hibit the  progressive  displacement ;  we  go  on  to  remark  that,  in 
the  case  of  the  pelvic  arch,  he  abandons  his  principle  of  appealing 
to  the  lowest  vertebrate  forms  for  proof  of  the  typical  structure. 
In  fishes,  the  rudimentary  pelvis,  widely  removed  from  the  spinal 
column,  shows  no  signs  of  having  belonged  to  any  vertebra ;  anfl 
nere  Professor  Owen  instances  the  percnnibrauchiate  HatraeJiia  as 


522 


exhibiting  the  typical  structure  :  remarking  that  "  mammals, 
and  reptiles  show  the  rule  of  connexion,  and  fishes  the  exception." 
Thus  in  the  case  of  the  scapular  arch,  the  evidence  afforded  by 
lishes  is  held  of  great  weight,  because  of  their  archetypal  regularity  ; 
while  in  the  case  of  the  pelvic  arch,  their  evidence  is  rejected  as 
exceptional.  But  now,  having,  as  he  considers,  shown  that  these 
bony  frames  to  which  the  limbs  are  articulated  are  modified  ha3inal 
arches,  Professor  Owen  points  out  that  the  haemal  arches  habitually 
bear  certain  "  diverging  appendages  ;"  and  he  aims  to  show  that 
the  "  diverging  appendages  "  of  the  scapular  and  pelvic  arches  re- 
spectively, are  developed  into  the  fore  and  hind  limbs.  There  are 
several  indirect  ways  in  which  we  may  test  the  probability  of  this 
conclusion.  If  these  diverging  appendages  are  "  rudimental  limbs  '"' 
—  "  future  possible  or  potential  arms,  legs,  wings,  or  feet,"  we  may 
fairly  expect  them  always  to  bear  to  the  hajmal  arches  a  relation 
such  as  the  limbs  do.  But  they  by  no  means  do  this.  "  As  the 
vertebra?  approach  the  tail,  these  appendages  are  often  transferred 
gradually  from  the  pleurapophysis  to  the  parapophysis,  or  even  to 
the  centrum  and  neural  arch."  (Arch,  and  Horn.,  p.  93.)  Again, 
it  might  naturally  be  assumed  that  in  the  lowest  vertebrate  forms, 
where  the  limbs  are  but  little  developed,  they  would  most  clearly 
display  then-  alliance  with  the  appendages,  or  "  rudimental  limbs," 
by  the  similarity  of  their  attachments.  Instead  of  this,  however, 
Professor  Owen's  drawings  show  that  whereas  the  appendages  are 
habitually  attached  to  the  pleurapophyses,  the  limbs,  in  their  earliest 
and  lowest  phase,  alike  in  fishes  and  in  the  Lepidosiren,  are  articu- 
lated to  the  haemapophyses.  Most  anomalous  of  all,  however,  is 
the  process  of  development.  When  we  speak  of  one  thing  as  being 
developed  out  of  another,  we  imply  that  the  parts  next  to  the  germ 
are  the  first  to  appear,  and  the  most  constant.  In  the  evolution  of 
a  tree  out  of  a  seed,  there  come  at  the  outset  the  stem  and  the 
radicle  ;  afterwards  the  branches  and  divergent  roots  ;  and  still 
later  the  branchlets  and  rootlets  ;  the  remotest  parts  being  the  latest 
and  most  inconstant.  If,  then,  a  limb  is  developed  out  of  a  "  di- 
verging appendage  "  of  the  ha3mal  arch,  the  earliest  and  most  con- 
stant bones  should  be  the  humerus  and  femur  ;  next  in  order  of 
time  and  constancy  should  come  the  coupled  bones  based  on  these  ; 
while  the  terminal  groups  of  bones  should  be  the  last  to  make  their 
appearance,  and  the  most  liable  to  be  absent.  Yet,  as  Professor 
Owen  himself  shows,  the'  actual  mode  of  development  is  the  very  re- 
verse of  this.  At  p.  16  of  the  Archetype  and  Ilomoloyies,  he  says:  — 

"  The  earlier  stages  in  the  development  of  all  locomotive  extremities  are 
permanently  retained  or  represented  in  the  paired  tins  of  fishes.  First  tha 
essential  part  of  the  member,  the  hand  or  foot,  appears  :  then  the  fore-arm 
u-  leg,  both  much  shortened,  flattened,  and  expanded,  as  in  all  fins  and  all 
embryonic  rudiments  of  limbs  :  finally  come  the  humeral  and  femoral  seg- 
ments ;  but  this  stage  I  have  not  found  attained  in  any  fish." 


523 

That  is  to  say,  alike  in  ascending  through  the  Vertebrata  gene- 
rally, and  in  tracing  up  the  successive  phases  of  a  mammalian  em- 
bryo, the  last-developed  and  least  constant  division  of  the  limb,  is 
that  basic  one  by  which  it  articulates  with  the  haemal  arch.  It 
seems  to  us  that,  so  far  from  proving  his  hypothesis,  Professor 
Owen's  own  facts  tend  to  show  that  limbs  do  not  belong  to  the 
vertebras  at  all ;  that  they  make  their  first  appearance  peripherally  ; 
that  their  development  is  centripetal ;  and  that  they  become  fixed 
to  such  parts  of  the  vertebrate  axis  as  the  requirements  of  the  case 
determine. 

But  now,  ending  here  this  digressive  exposition  and  criticism, 
and  granting  the  position  that  limbs  "  are  developments  of  costal 
appendages,"  let  us  return  to  the  question  above  put — Why  are  not 
these  appendages  included  as  elements  of  the  "  ideal  typical  ver- 
tebra ?  "  It  cannot  be  because  of  their  comparative  inconstancy  ; 
for  judging  from  the  illustrative  figures,  they  seem  to  be  as  con- 
stant as  the  haemal  spine,  which  is  one  of  the  so-called  autogenous 
elements  :  in  the  diagram  of  the  Archetypus,  the  appendage  is  re- 
presented as  attached  to  every  vertebrate  segment  of  the  head  and 
trunk,  which  the  haemal  spine  is  not.  It  cannot  be  from  their  com- 
parative unimportance ;  seeing  that  as  potential  limbs  they  are 
essential  parts  of  nearly  all  the  Vertebrata — much  more  obviously 
so  than  the  diapophyses  are.  If,  as  Professor  Owen  argues,  "  the 
divine  mind  which  planned  the  archetype  also  foreknew  all  its 
modifications  ;"  and  if,  among  these  modifications,  the  development 
of  limbs  out  of  diverging  appendages  was  one  intended  to  charac- 
terize all  the  higher  Vertebrata ;  then,  surely,  these  diverging  ap- 
pendages must  have  been  parts  of  the  "  ideal  typical  vertebra." 
Or,  if  the  "  ideal  typical  vertebra"  is  to  be  understood  as  a  crystal- 
line form  in  antagonism  with  the  organizing  principle  ;  then  why 
should  not  the  appendages  be  included  among  its  various  offshoots  ? 
We  do  not  ask  this  question  because  of  its  intrinsic  importance. 
Wre  ask  it  for  the  purpose  of  ascertaining  Professor  Owen's  method 
of  determining  what  are  true  vertebral  constituents.  He  presents 
us  with  a  diagram  of  the  typical  vertebra,  in  which  are  included 
certain  bones,  and  from  which  are  excluded  certain  others.  If  re- 
lative constancy  is  the  criterion,  then  there  arises  the  question — • 
What  degree  of  constancy  entitles  a  bone  to  be  included  ?  If  re- 
lative importance  is  the  criterion,  there  comes  not  only  the  question 
— What  degree  of  importance  suffices  ?  but  the  further  question 
— How  is  importance  to  be  measured  ?  If  neither  of  these  is  the 
criterion,  then  what  is  it  ?  And  if  there  is  no  criterion,  does  it 
not  follow  that  the  selection  is  arbitrary  ? 

This  question  serves  to  introduce  a  much  wider  one  : — Has  the 
'  ideal  typical  vertebra  "  any  essential  constituents  at  all  ?    It  might 


524 

naturally  he  supposed  that  though  some  bones  are  so  rarely 
developed  as  not  to  seem  worth  including,  and  though  some  that 
are  included  are  very  apt  to  be  absent  j  yet  that  certain  others  are 
invariable :  forming,  as  it  were,  the  basis  of  the  ideal  type.  Let 
us  see  whether  the  facts  bear  out  this  supposition.  In  his  "summary 
of  modifications  of  corporal  vertebrae "  (p.  96),  Professor  Owen 
says — "  The  hcemal  spine  is  much  less  constant  as  to  its  existence, 
and  is  subject  to  a  much  greater  range  of  variety,  when  present, 
than  its  vertical  homotype  above,  which  completes  the  neural  arch." 
Again  he  says — "  The  hcemapophyses,  as  osseous  elements  of  a 
vertebra,  are  less  constant  than  the  pleurapophyses."  And  again — 
"  The  pleurapaphyses  are  less  constant  elements  than  the  neurapo- 
physes."  And  again — "Amongst  air-breathing  vertebrates  the 
pleurapophyses  of  the  trunk  segments  are  present  only  in  those  species 
in  which  the  septum  of  the  heart's  ventricle  is  complete  and  imper- 
forate,  and  here  they  are  exogenous  and  confined  to  the  cervical 
and  anterior  thoracic  vertebrae."  And  once  more,  both  the  neura- 
pophyses  and  the  neural  spine  ;'  are  absent  under  both  histological 
conditions,  at  the  end  of  the  tail  in  most  air-breathing  vertebrates, 
where  the  segments  are  reduced  to  their  central  elements."  That 
is  to  say,  of  all  the  peripheral  elements  of  the  "ideal  typical 
vertebra,"  there  is  not  one  which  is  always  present.  It  will  be  ex- 
pected, however,  that  at  any  rate  the  centrum  is  constant :  the  bone 
which  "  forms  the  axis  of  the  vertebral  column,  and  commonly  the 
central  bond  of  union  of  the  peripheral  elements  of  the  vertebra 
(p.  97),  is  of  course  an  invariable  element.  No  :  not  even  this  is 
essential. 

"The  centrums  do  not  pass  beyond  the  primitive  stage  of  the  notochord 
(undivided  column)  in  the  existing  lepidosiren,  and  they  retained  the  like 
rudimental  state  in  every  fish  whose  remains  have  been  found  in  strata 
earlier  than  the  perniian  aera  in  Geology,  though  the  number  of  vertebra?  is 
frequently  indicated  in  Devonian  and  Silurian  ichthyolites  by  the  fossilized 
neur-  and  hsemapophyses  and  their  spines  "  (p.  96). 

Indeed,  Professor  Owen  himself  remarks  that  "the  neurapo- 
physes  are  more  constant  as  osseous  or  cartilaginous  elements  of  the 
vertebra?  than  the  centrums"  (p.  97).  Thus,  then,  it  appears  that 
the  several  elements  included  in  the  "  ideal  typical  vertebra  '*  have 
various  degrees  of  constancy,  and  that  no  one  of  them  is  essential. 
There  is  no  one  part  of  a  vertebra  which  invariably  answers  to  its 
exemplar  in  the  pattern-group.  How  does  this  fact  consist  with  the 
hypothesis  ?  If  the  Creator  saw  fit  to  make  the  vertebrate  skeleton 
out  of  a  series  of  segments,  all  formed  on  essentially  the  same  model 
— if,  for  the  maintenance  of  the  type,  one  of  these  bony  segments  is 
in  many  cases  formed  out  of  a  coalesced  group  of  pieces,  where,  as 
Professor  Owen  argues,  a  single  piece  would  have  served  as  well  or 
better  ;  than  we  ought  to  find  this  typical  repetition  of  parts  oiii- 


525 

formly  manifested.  Without  any  change  of  shape,  it  would  obri 
ously  have  been  quite  possible  for  every  actual  vertebra  to  hare 
contained  all  the  parts  of  the  ideal  one — ruclimentally  where  they 
were  not  wanted.  Even  oae  of  the  terminal  bones  of  a  mammal's 
tail  might  have  been  formed  out  of  the  nine  autogenous  pieces, 
united  by  suture  but  admitting  of  identification.  As,  however, 
there  is  no  such  uniform  typical  repetition  of  parts,  it  seem?  to  us 
that  to  account  for  the  typical  repetition  which  does  occur,  by  sup- 
posing the  Creator  to  have  fixed  on  a  pattern-vertebra,  is  to  ascribe 
to  him  the  inconsistency  of  forming  a  plan  and  then  abandoning  it. 
If,  on  the  other  hand,  Professor  Owen  means  that  the  "  ideal 
typical  vertebra"  is  a  crystalline  form  in  antagonism  with  "  the 
idea  or  organizing  principle  ;"  then  we  might  fairly  expect  to  find 
it  most  clearly  displaying  its  crystalline  character,  and  its  full  com- 
plement of  parts,  in  those  places  where  the  organizing  principle 
may  be  presumed  to  have  "  subdued "  it  to  the  smallest  extent. 
Yet  in  the  Vertebrata  generally,  and  even  in  Professor  Owen's 
Archetypus,  the  vertebras  of  the  tail,  which  must  be  considered  as, 
if  anything,  less  under  the  influence  of  the  organizing  principle 
than  those  of  the  trunk,  do  not  manifest  the  ideal  form  more  com- 
pletely. On  the  contrary,  as  we  approach  the  end  of  the  tail,  the 
successive  segments  not  only  lose  their  remaining  typical  elements, 
but  become  as  uncrystalline-looking  as  can  be  conceived. 

Supposing,  however,  that  the  assumption  of  suppressed  or  unde- 
veloped elements  be  granted — supposing  it  to  be  consistent  with 
the  hypothesis  of  an  "  ideal  typical  vertebra,"  that  the  constituent 
parts  may  severally  be  absent  in  greater  or  less  number,  sometimes 
leaving  only  a  single  bone  to  represent  them  all ;  may  it  not  be  that 
such  parts  as  are  present,  show  their  respective  typical  natures  by 
some  constant  character :  say  their  mode  of  ossification  1 

To  this  question  some  parts  of  the  Archetype  and  Homologies  seem 
to  reply,  "  Yes ;"  while  others  clearly  answer,  "  No."  Criticising 
the  opinions  of  Geoffrey  St.  Hilaire  and  Cuvier,  who  agreed  in 
thinking  that  ossification  from  a  separate  centre  was  the  test  of  a 
separate  bone,  and  that  thus  there  were  as  many  elementary  bones 
in  the  skeleton  as  there  were  centres  of  ossification,  Professor  Owen 
points  out  that,  according  to  this  test,  the  human  femur,  which  is 
ossified  from  four  centres,  must  be  regarded  as  four  bones  ;  while 
the  femur  in  birds  and  reptiles,  which  is  ossified  from  a  single 
centre,  must  be  regarded  as  a  single  bone.  Yet,  on  the  other  hand, 
he  attaches  weight  to  the  fact  that  the  skull  of  the  human  foetus 
presents  "  the  same  ossific  centres  "  as  do  those  of  the  embryo  kan- 
garoo and  the  young  bird.  (Nature  of  Limbs,  p.  40.)  And  at  p. 
104  of  the  Homologies,  after  giving  a  number  of  instances,  he  says — 
•  These  and  the  like  correspondences  between  the  points  of  ossification  ot 


526 

the  human  foetal  skeleton,  and  the  separate  bones  of  the  adult  skeletons  of 
inferior  animals,  are  pregnant  with  interest,  and  rank  among  the  most  striking 
illustrations  of  unity  of  plan  in  the  vertebrate  organization." 

It  is  true  that  on  the  following  page  he  seeks  to  explain  this 
seeming  contradiction  by  distinguishing 

"between  those  centres  of  ossification  that  have  homological  relations,  and 
those  that  have  teleological  ones— i.e.,  between  the  separate  points  of  ossifica- 
tion of  a  human  bone  which  typify  vertebral  elements,  often  permanently  dis- 
tinct bones  in  the  lower  animals  ;  and  the  separate  points  which,  without  such 
signification,  facilitate  the  progress  of  osteogeriy,  and  have  for  their  obvious 
linal  cause  the  well-being  of  the  growing  animal." 

But  if  there  are  thus  centres  of  ossification  which  have  homo- 
logical  meanings,  and  others  which  have  not,  there  arises  the  ques- 
tion— How  are  they  always  to  be  distinguished  ?  Evidently  in- 
dependent ossification  ceases  to  be  a  homological  test,  if  there  are 
independent  ossifications  that  have  nothing  to  do  with  the  homo- 
logies.  And  this  becomes  the  more  evident  when  we  learn  that 
there  are  cases  where  neither  a  homological  nor  a  teleological 
meaning  can  be  given.  Among  various  modes  of  ossification  of  the 
centrum,  Professor  Owen  points  out  that  "  the  body  of  the  human 
atlas  is  sometimes  ossified  from  two,  rarely  from  three,  distinct 
centres  placed  side  by  side  "  (p.  89)  ;  while  at  p.  87  he  says  : — "  In 
osseous  fishes  I  find  that  the  centrum  is  usually  ossified  from  six 
points."  It  is  clear  that  this  mode  of  ossification  has  here  no  homo- 
logical  signification ;  and  it  would  be  difficult  to  give  any  teleo- 
logical reason  why  the  small  centrum  of  a  fish  should  have  more 
centres  of  ossification  than  the  large  centrum  of  a  mammal.  The 
truth  is,  that  as  a  criterion  of  the  identity  or  individuality  of  a  bone, 
mode  of  ossification  is  quite  untrustworthy.  Though,  in  his  "  ideal 
typical  vertebra,"  Professor  Owen  delineates  and  classifies  as  sepa- 
rate' "  autogenous  "  elements,  those  parts  which  are  "  usually 
developed  from  distinct  and  independent  centres ;"  and  though  by 
doing  so  he  erects  this  characteristic  into  some  sort  of  criterion  ; 
yet  his  own  facts  show  it  to  be  no  criterion.  The  parapophyses 
are  classed  among  the  autogenous  elements ;  yet  they  are  auto- 
genous in  fishes  alone,  and  in  these  only  in  the  trunk  vertebra, 
while  in  all  air-breathing  vertebrates  they  are,  when  present  at  all, 
exogenous.  The  neurapophyses,  again,  "  lose  their  primitive  in- 
dividuality by  various  kinds  and  degrees  of  confluence:"  in  the 
tails  of  the  higher  Vertebrata  they,  in  common  with  the  neural 
spine,  become  exogenous.  Nay,  even  the  centrum  may  lose  its 
autogenous  character.  Describing  how,  in  some  batrachiaus, 
"  the  ossification  of  the  centrum  is  completed  by  an  extension  of 
bone  from  the  bases  of  the  neurapophyses,  which  effects  also  the 
coalescence  of  these  with  the  centrum,"  Professor  Owen  adds  : — > 
"  In  Pelobates  fuscus  and  Pelobates  cultripes.  Mliller  found  the  en- 


527 

tire  centrum  ossified  .from  this  source,  without  any  independent 
points  of  ossification  "  (p.  88).  That  is  to  say,  the  centrum  is  in 
these  cases  an  exogenous  process  of  the  neurapophyses.  We  see, 
then,  that  these  so-called  typical  elements  of  vertebne  have  no 
constant  developmental  character  by  which  they  can  be  identified. 
Not  only  are  they  undistinguishable  by  any  specific  test  from  other 
bones  not  included  as  vertebral  elements  ;  not  only  do  they  fail  to 
show  their  typical  characters  by  their  constant  presence ;  but, 
when  present,  they  exhibit  no  persistent  marks  of  individuality. 
The  central  element  may  be  ossified  from  six,  four,  three,  or  two 
points ;  or  it  may  have  no  separate  point  of  ossification  at  all : 
and  similarly  with  various  of  the  peripheral  elements.  The  whole 
group  of  bones  forming  the  "  ideal  typical  vertebra"  may  severally 
have  their  one  or  more  ossific  centres  ;  or  they  may,  as  in  a  mam- 
mal's tail,  lose  their  individualities  in  a  single  bone  ossified  from 
one  or  two  points. 

Another  fact  which  seems  very  difficult  to  reconcile  with  the 
hypothesis  of  an  "  ideal  typical  vertebra,"  is  the  not  infrequent 
presence  of  some  of  the  typical  elements  in  duplicate.  Not  only, 
as  we  have  seen,  may  they  severally  be  absent ;  but  they  may  seve- 
rally be  present  in  greater  number  than  they  should  be.  When  we 
see,  in  the  ideal  diagram,  one  centrum,  two  neurapophyses,  two 
pleurapophyses,  two  haemapophyses,  one  neural  spine,  and  one 
hasmal  spine,  we  naturally  expect  to  find  them  always  bearing  to 
each  other  these  numerical  relations.  Though  we  may  not  be 
greatly  surprised  by  the  absence  of  some  of  them,  we  are  hardly 
prepared  to  find  others  multiplied.  Yet  such  cases  are  common. 
Thus  the  neural  spine  "  is  double  in  the  anterior  vertebra?  of  somo 
fishes  "  (p.  98).  Again,  in  the  abdominal  region  of  extinct  saurians, 
and  in  crocodiles,  "  the  freely-suspended  hremapophyses  are  com- 
pounded of  two  or  more  overlapping  bony  pieces  "  (p.  100).  Yet 
again,  at  p.  99,  we  read — "  I  have  observed  some  of  the  expanded 
pleurapophyses  in  the  great  Testudo  elephantopus  ossified  from  two 
centres,  and  the  resulting  divisions  continuing  distinct,  but  united 
by  suture."  Once  more  "  the  neurapophyses,  which  do  not  advance 
beyond  the  cartilaginous  stage  in  the  sturgeon,  consist  in  that  fish 
of  two  distinct  pieces  of  cartilage  ;  and  the  anterior  pleurapophyses 
also  consist  of  two  or  more  cartilages,  set  end  on  end"  (p.  91). 
And  elsewhere  referring  to  this  structure,  he  says  : — 

"  Vegetative  repetition  of  peri  vertebral  parts  not  only  manifests  itself  in 
the  composite  neurapophyses  and  pleurapophyses,  but  in  a  small  accessory 
(interneural)  cartilage,  at  the  fore  and  back  part  of  the  base  of  the  neura- 
Dophysis  ;  and  by  a  similar  (interhtemal)  one  at  the  fore  and  back  part  of 
most  of  the  parapophyses  "  (p.  87). 

Thus  the  neural  and  haemal  spines,  the.  neurapophyses,  the  plea- 
Vo:..  II.  S3 


523 

rapophyses,  the  hsemapophyses,  may  severally  consist  of  two  or  more 
pieces.     This  is  not  all :  the  like  is  true  even  of  the  centrums. 

"  In  Heptanchus  (Squaliis  clnereus]  the  vertebral  centres  are  feebly  aii'J 
vegetatively  marked  out  by  numerous  slender  rings  of  hard  cartilage  in  the 
notochordal  capsule,  the  number  of  vertebrae  being  more  definitely  indicated 
by  the  neurapophyses  and  .  parapophyses.  ...  In  the  piked  dog-iish 
(Acanthias)  and  the  spotted  dog-tish  (Scylliurn)  the  vertebral  centres  coin- 
cide in  number  with  the  neural  arches  "  (p.  87). 

Is  it  not  strange  that  the  pattern  vertebra  should  be  so  little  ad- 
hered to,  that  each  of  its  single  typical  pieces  may  be  transformed 
into  two  or  three  ? 

But  there  are  still  more  startling  departures  from  the  alleged 
type.  The  numerical  relations  of  the  elements  vary  not  only  in 
this  way,  but  in  the  opposite  way.  A  given  part  may  be  present 
not  only  in  greater  number  than  it  should  be,  but  also  in  less.  In 
the  tails  of  homocercal  fishes,  the  centrums  "  are  rendered  by  cen- 
tripetal shortening  and  bony  confluence  fewer  in  number  than  the 
persistent,  neural,  and  haemal  arches  of  that  part " — that  is,  there 
is  only  a  fraction  of  a  centrum  to  each  vertebra.  Nay,  even  this 
is  not  the  most  heteroclite  structure.  Paradoxical  as  it  may  seem, 
there  are  cases  in  which  the  same  vertebral  element  is,  considered 
under  different  aspects,  at  once  in  excess  and  defect.  Speaking  of 
the  hasmal  spine,  Professor  Owen  says  : — 

"  The  horizontal  extension  of  this  vertebral  element  is  sometimes  accom- 
panied by  a  median  division,  or  in  other  words,  it  is  ossitied  from  two 
lateral  centres  ;  this  is  seen  in  the  development  of  parts  of  the  human 
sternum  ;  the  same  vegetative  character  is  constant  in  the  broader  thoracic 
haiinal  spines  of  birds  ;  though,  sometimes,  as  e.g.,  in  the  struthionidfe, 
oxfiijication  extends  from  the  same  lateral  centre  l".nijt  Incite— i.e.,  forwards  and 
backwards,  calcifying  the  connate  cartilaginous  homologucs  of  halves  of  four 
or  fioe  Juvmal  spinet,  before  these  finally  coalesce  with  their  fellows  at  the 
median  line  "  (p.  101). 

So  that  the  sternum  of  the  ostrich,  which  according  to  the  hypo- 
thesis, should,  in  its  cartilaginous  stage,  have  consisted  of  four  or 
five  transverse  pieces,  answering  to  the  vertebral  segments,  and 
should  have  been  ossified  from  four  or  five  centres,  one  to  each 
cartilaginous  piece,  shows  not  a  trace  of  this  structure;  but  in- 
stead, consists  of  two  longitudinal  pieces  of  cartilage,  each  ossified 
from  one  centre,  and  finally  coalescing  on  the  median  line.  These 
four  or  five  haemal  spines  have  at  the  same  time  doubled  their  in- 
dividualities transversely,  and  entirely  lost  them  longitudinally ! 

There  still  remains  to  be  considered  the  test  of  relative  position. 
It  might,  be  held  that,  spite  of  all  the  foregoing  anomalies,  if  the 
typical  parts  of  the  vertebrae  always  stood  towards  each  other  in 
the  same  relations — always  preserved  the  same  connexions,  some- 
thing like  a  case  would  be  made  out.  Doubtless,  relative  position 


529 

is  an  important  point ;  and  it  is  one  on  which  Professor  Owen  mani- 
festly places  great  dependence.  In  his  discussion  of  "  moot  cases 
of  special  homology,"  it  is  the  general  test  to  which  he  appeals. 
The  typical  natures  of  the  alisphenoid,  the  mastoid,  the  orbito- 
sphenoid,  the  prefrontal,  the  malar,  the  squamosal,  &c.,  he  deter- 
mines almost  wholly  by  reference  to  the  adjacent  nerve-perforation* 
and  the  articulations  with  neighbouring  bones  (see  pp.  19  to  72) : 
the  general  form  of  the  argument  being — This  bone  is  to  be  classed 
as  such  or  such,  because  it  is  connected  thus  and  thus  with  these 
others,  which  are  so  and  so.  Moreover,  by  putting  forth  an  "  ideal 
typical  vertebra,"  consisting  of  a  number  of  elements  standing 
towards  each  other  in  certain  definite  arrangement,  this  persistency 
of  relative  position  is  manifestly  alleged.  The  essential  attribute 
of  this  group  of  bones,  considered  as  a  typical  group,  is  the  con- 
stancy in  the  connexions  of  its  parts  :  change  the  connexions,  and 
the  type  is  changed.  But  the  constancy  of  relative  position  thus 
tacitly  asserted,  and  appealed  to  as  a  conclusive  test  in  "  moot 
cases  of  special  homology,"  is  clearly  negatived  by  Professor 
Owen's  own  facts.  For  instance,  in  the  "  ideal  typical  vertebra," 
the  hremal  arch  is  represented  as  formed  by  the  two  basmapophyses 
and  the  haemal  spine ;  but  at  p.  91  we  are  told  that 

"  The  contracted  haemal  arch  in  the  caudal  region  of  the  body  may  be 
formed  by  different  elements  of  the  typical  vertebra:  e.g.,  by  the  para-- 
pophyses  (fishes  generally)  ;  by  the  pleurapophyses  (lepidosiren)  ;  by  both 
parapophyses  and  pleurapophyses  (tiudis,  Lepidoateus),  and  by  hsemapo- 
physes,  shortened  and  directly  articulated  with  the  centrums  (reptiles  and 
inammals)." 

And  further,  in  the  thorax  of  reptiles,  birds,  and  mammals,  "  the 
hsemapophyses  are  removed  from  the  centrum,  and  are  articulated  to 
the  distal  ends  of  the  pleurapophyses ;  the  bony  hoop  being  com- 
pleted by  the  intercalation  of  the  haemal  spine  "  (p.  82).  So  that 
there  are/a'e  different  ways  in  which  the  hannal  arch  may  be  formed 
— four  modes  of  attachment  of  the  parts  different  from  that  shown 
in  the  typical  diagram !  Nor  is  this  all.  The  pleurapophyses  "  may 
be  quite  detached  from  their  proper  segment,  and  suspended  to  the 
haemal  arch  of  another  vertebra ;"  as  we  have  already  seen,  the 
entire  hyemal  arch  may  be  detached  and  removed  to  a  distance, 
sometimes  reaching  the  length  of  twenty-seven  vertebras  ;  and,  even 
more  remarkable,  the  ventral  fins  of  some  fishes,  which  theoretically 
belong  to  the  pelvic  arch,  are  so  much  advanced  forward  as  to  bo 
articulated  to  the  scapular  arch — "  the  ischium  elongating  to  joiu 
the  coracoid."  With  these  admissions  it  seems  to  us  that  relative 
positipn  and  connexions  cannot  be  appealed  to  as  tests  of  homology, 
oor  as  evidence  of  any  original  type  of  vertebra. 

In  no  class  of  facts,  then,  do  we  find  a  good  foundation  for  the 
hypothesis  of  an  "  ideal  typical  vertebra."  There  is  no  one  coa- 


680 

ceivable  attribute  of  this  archetypal  form  which  is  habitually  realised 
by  actual  vertebrae.  The  alleged  group  of  true  vertebral  elements 
is  not  distinguished  in  any  specified  way  from  bones  not  included  in 
it.  Its  members  have  various  degrees  of  inconstancy ;  are  rarely 
all  present  together ;  and  no  one  of  them  is  essential.  They  are 
severally  developed  in  no  uniform  way :  each  of  them  may  "arise 
either  out  of  a  separate  piece  of  cartilage,  or  out  of  a  piece  con- 
tinuous with  that  of  some  other  element ;  and  each  may  be  ossified 
from  many  independent  points,  from  one,  or  from  none.  Not  only 
may  their  respective  individualities  be  lost  by  absence,  or  by  con- 
fluence with  others ;  but  they  may  be  doubled,  or  tripled,  or  halved, 
or  may  be  multiplied  in  one  direction  and  lost  in  another.  The  en- 
tire group  of  typical  elements  may  coalesce  into  one  simple  bone 
representing  the  whole  vertebra  ;  and  even,  as  in  the  terminal  piece 
of  a  bird's  tail,  half-a-dozen  vertebrae,  with  all  their  many  elements, 
may  become  entirely  lost  in  a  single  mass.  Lastly,  the  respective 
elements,  when  present,  have  no  fixity  of  relative  position  :  sundry 
of  them  are  found  articulated  to  various  others  than  those  with 
which  they  are  typically  connected ;  they  are  frequently  displaced 
and  attached  to  neighbouring  vertebrae  ;  and  they  are  even  removed 
to  quite  remote  parts  of  the  skeleton.  It  seems  to  us  that  if  this 
want  of  congruity  with  the  facts  does  not  disprove  the  hypothesis, 
no  such  hypothesis  admits  of  disproof. 

Unsatisfactory  as  is  the  evidence  in  the  case  of  the  trunk  and 
tail  vertebrae,  to  which  we  have  hitherto  confined  ourselves,  it  is  far 
worse  in  the  case  of  the  alleged  cranial  vertebrae.  The  mere  fact 
that  those  who  have  contended  for  the  vertebrate  structure  of  the 
skull,  have  differed  so  astonishingly  in  their  special  interpretations 
of  it,  is  enough  to  warrant  great  doubt  as  to  the  general  truth  of 
their  theory.  From  Professor  Owen's  history  of  the  doctrine  of 
general  homology,  we  gather  that  Dumeril  wrote  upon  "  la  tc'te 
considerec  comme  une  vertebre ;"  that  Kielmeyer,  "  instead  of 
calling  the  skull  a  vertebra,  said  each  vertebra  might  be  called  a 
skull;"  that  Oken  recognized  in  the  skull  three  vertebrae  and  a 
rudiment ;  that  Professor  Owen  himself  makes  out  four  vertebrae ; 
that  Goethe's  idea,  adopted  and  developed  by  Carus,  was,  that  the 
skull  is  composed  of  six  vertebras ;  and  that  Geoffrey  St.  Hilaire 
divided  it  into  seven.  Does  not  the  fact  that  different  comparative 
anatomists  have  arranged  the  same  group  of  bones  into  one,  three, 
four,  six,  and  seven  vertebral  segments,  show  that  the  mode  of  de- 
termination is  arbitrary,  and  the  conclusions  arrived  at  fanciful  ? 
May  we  not  properly  entertain  great  doubts  as  to  any  one  scheme 
being  more  valid  than  the  others  ?  And  if  out  of  these  conflicting 
schemes  we  are  asked  to  accept  one,  ought  we  not  to  accept  it  only 
on  the  production  of  some  thoroughly  conclusive  proof — soraft 


581 

rigorous  test  showing  irrefragably  that  the  others  must  be  wrong 
and  this  alone  right  ?  Evidently  where  such  contradictory  opinions 
have  been  formed  by  so  many  competent  judges,  we  ought,  before 
deciding  in  favour  of  one  of  them,  to  have  a  clearness  of  demon- 
stration much  exceeding  that  required  in  any  ordinary  case.  Let 
us  see  whether  Professor  Owen  supplies  us  with  any  such  clearness 
of  demonstration. 

To  bring  the  first  or  occipital  segment  of  the  skull  into  corre- 
spondence with  the  "  ideal  typical  vertebra,"  Professor  Owen  argues, 
in  the  case  of  the  fish,  that  the  parapophyses  are  displaced,  and 
wedged  between  the  neurapophyses  and  the  neural  spine — removed 
from  the  haemal  arch  and  built  into  the  upper  part  of  the  neural 
arch.  Further,  he  considers  that  the  pleurapophyses  are  teleologi- 
cally  compound.  And  then,  in  all  the  higher  vertebrata,  he  alleges 
that  the  haemal  arch  is  separated  from  its  centrum,  taken  to  a  dis- 
tance, and  transformed  into  the  scapular  arch.  Add  to  which,  he 
says  that  in  mammals  the  displaced  parapophyses  are  mere  processes 
of  the  neurapophyses  (p.  133) :  these  vertebral  elements,  typically 
belonging  to  the  lower  part  of  the  centrum,  and  in  nearly  all  cases 
confluent  with  it,  are  not  only  removed  to  the  far  ends  of  elements 
placed  above  the  centrum,  but  have  become  exogenous  parts  of  them  ! 

Conformity  of  the  second  or  parietal  segment  of  the  cranium  with 
the  pattern-vertebra,  is  produced  thus : — The  petrosals  are  excluded 
as  being  partially-ossified  sense-capsules,  not  forming  parts  of  the 
true  vertebral  system,  but  belonging  to  the  "  splanchuo-skeleton." 
A  centrum  is  artificially  obtained  by  sawing  in  two  the  bone  which 
serves  in  common  as  centrum  to  this  and  the  preceding  segment ;  and 
this  though  it  is  admitted  that  in  fishes,  where  their  individualities 
ought  to  be  best  seen,  these  two  hypothetical  centrums  are  not 
simply  coalescent,  but  connate.  Next,  a  similar  arbitrary  bisection 
is  made  of  certain  elements  of  the  haemal  arches.  And  then,  "  the 
principle  of  vegetative  repetition  is  still  more  manifest  in  this  arch 
than  in  the  occipital  one :"  each  pleurapophysis  is  double ;  each 
Imnapophysis  is  double ;  and  the  haemal  spine  consists  of  six  pieces  ! 

The  interpretation  of  the  third  and  fourth  segments  being  of  the 
same  general  character,  need  not  be  detailed.  The  only  point 
calling  for  remark  being,  that  in  addition  to  the  above  various 
modes  of  getting  over  anomalies,  we  find  certain  bones  referred  to 
the  dermo-skeleton. 

Now  it  seems  to  us,  that  even  supposing  no  antagonist  interpre- 
tations bad  been  given,  an  hypothesis  reconcilable  with  the  facts 
only  by  the  aid  of  so  many  questionable  devices,  could  not  be  con- 
sidered satisfactory  ;  and  that  when,  as  in  this  case,  various  com- 
parative anatomists  have  contended  for  other  interpretations,  the 
character  of  this  one  is  certainly  not  of  a  kind  to  warrant  the  re- 
jection of  the  others  in  its  favour;  but  rather  of  a  kind  to  mak« 


582 

ns  doubt  the  possibility  of  all  such  interpretations.  The  question 
which  naturally  arises  is,  whether  by  proceeding  after  this  fashion, 
groups  of  bones  might  not  be  arranged  into  endless  typical  forms. 
If,  when  a  given  element  was  not  in  its  place,  we  were  at  liberty  to 
consider  it  as  suppressed,  or  connate  with  some  neighbouring  element, 
or  removed  to  some  more  or  less  distant  position  ; — if,  on  finding  a 
bone  in  excess,  we  might  consider  it,  now  as  part  of  the  dermc- 
skeleton,  now  as  part  of  the  splanchno-skeleton,  now  as  transplanted 
from  its  typical  position,  now  as  resulting  from  vegetative  repetition, 
and  now  as  a  bone  Ideologically  compound  (for  these  last  two  are 
intrinsically  different,  though  often  used  by  Professor  Owen  as 
equivalents); — if,  in  other  cases,  a  bone  might  be  regarded  as 
spurious  (p.  91),  or  again  as  having  usurped  the  place  of  another  ; — 
if,  we  say,  these  various  liberties  were  allowed  us,  we  should  not 
despair  of  reconciling  the  facts  with  various  diagrammatic  types 
besides  that  adopted  by  Professor  Owen. 

When,  in  1851,  we  attended  a  course  of  Professor  Owen's  lectures 
on  Comparative  Osteology,  beginning  though  we  did  in  the  attitude 
of  discipleship,  our  scepticism  grew  as  we  listened,  and  reached  its 
climax  when  we  came  to  the  skull ;  the  reduction  of  which  to  the 
vertebrate  structure,  reminded  us  very  much  of  the  interpretation 
of  prophecy.  The  delivery,  at  the  Royal  Society,  of  the  Croonian 
Lecture  for  1858,  in  which  Professor  Huxley,  confirming  the  state- 
ments of  several  German  anatomists,  has  shown  that  the  facts  of 
embryology  do  not  countenance  Professor  Owen's  views  respecting 
the  formation  of  the  cranium,  has  induced  us  to  reconsider  the  verte- 
bral theory  as  a  whole.  Closer  examination  of  Professor  Owen's 
doctrines,  as  set  forth  in  his  works,  has  certainly  not  removed  the 
scepticism  generated  years  ago  by  his  lectures.  On  the  contrary, 
that  scepticism  has  deepened  into  disbelief.  And  we  venture  to  think 
that  the  evidence  above  cited  shows  this  disbelief  to  be  warranted. 

There  remains  the  question — What  general  views  are  we  to  take 
respecting  the  vertebrate  structure  ?  If  the  hypothesis  of  an  "  ideal 
typical  vertebra"  is  not  justified  by  the  facts,  how  are  we  to  under- 
stand that  degree  of  similarity  which  vertebrae  display  ? 

We  believe  the  explanation  is  not  far  to  seek.  All  that  our  space 
will  here  allow,  is  a  brief  indication  of  what  seems  to  us  the  natura. 
view  of  the  matter. 

Professor  Owen,  in  common  with  other  comparative  anatomists, 
regards  the  divergences  of  individual  vertebra?  from  the  average 
form,  as  due  to  adaptive  modifications.  If  here  one  vertebral  ele 
ment  is  largely  developed,  while  elsewhere  it  is  small — if  now  the 
form,  now  the  position,  now  the  degree  of  coalescence,  of  a  given 
part  varies ;  -it  is  that  the  local  requirements  have  involved  this 
change.  The  entire  teaching  of  comparative  osteology  implies  that 


533 

differences  in  the  conditions  of  the  respective  vertebrae  necessitate 
differences  in  their  structures. 

Now,  it  seems  to  us  that  the  first  step  towards  a  right  conception 
of  the  phenomena,  is  to  recognize  this  general  law  in  its  converse 
application.  If  vertebrae  are  unlike  in  proportion  to  the  unlikeness 
of  their  circumstances,  then,  by  implication,  they  will  be  like  in  pro- 
portion to  the  likeness  of  their  circumstances.  While  successive 
segments  of  the  same  skeleton,  and  of  different  skeletons,  are  all  in 
eonie  respects  more  or  less  differently  acted  on  by  incident  forces, 
and  are  therefore  required  to  be  more  or.  less  different ;  they  are 
all,  in  other  respects,  similarly  acted  on  by  incident  forces,  and  are 
therefore  required  to  be  more  or  less  similar.  It  is  impossible  to 
deny  that  if  differences  in  the  mechanical  functions  of  the  vertebras 
involve  differences  in  their  forms ;  then,  community  in  their  mechani- 
cal functions,  must  involve  community  in  their  forms.  And  as  we 
know  that  throughout  the  Vertebrata  generally,  and  in  each  vertebrate 
animal,  the  vertebras,  amid  all  their  varying  circumstances,  have  a 
certain  community  of  function,  it  follows  necessarily  that  they  will 
have  a  certain  general  resemblance — there  will  recur  that  average 
shape  which  has  suggested  the  notion  of  a  pattern  vertebra. 

A  glance  at  the  facts  at  once  shows  their  harmony  with  this 
conclusion.  In  an  eel  or  a  snake,  where  the  bodily  actions  are  such 
as  to  involve  great  homogeneity  in  the  mechanical  conditions  of  tho 
vertebrae,  the  series  of  them  is  comparatively  homogeneous.  On  the 
contrary,  in  a  mammal  or  a  bird,  where  there  is  considerable  hetero- 
geneity in  their  circumstances,  their  similarity  is  no  longer  so  great. 
And  if,  instead  of  comparing  the  vertebral  columns  of  different 
animals,  we  compare  the  successive  vertebras  of  any  one  animal,  we 
recognize  the  same  law.  In  the  segments  of  an  individual  spine, 
where  is  there  the  greatest'  divergence  from  the  common  mechanical 
conditions  ?  and  where  may  Ave  therefore  expect  to  find  the  widest 
departure  from  the  average  form  ?  Obviously  at  the  two  extremities. 
And  accordingly  it  is  at  the  two  extremities  that  the  ordinary  struc- 
ture is  lost. 

Still  clearer  becomes  the  truth  of  this  view,  when  we  consider  the 
genesis  of  the  vertebral  column  as  displayed  throughout  the  ascend- 
ing grades  of  the  Vertebrata.  In  its  first  embryonic  stage,  the  soine 
is  an  undivided  column  of  flexible  substance.  In  the  early  fishes, 
while  some  of  the  peripheral  elements  of  the  vertebrae  were  marked 
out,  the  central  axis  was  still  a  continuous  unossified  cord.  And 
thus  we  have  good  reason  for  thinking  that  in  the  primitive  verte- 
brate animal,  as  in  the  existing  Amphinxus,  the  notochord  was  per- 
sistent. The  production  of  a  higher,  more  powerful,  more  active 
creature  of  the  same  type,  by  whatever  method  it  is  conceived  to 
have  taken  place,  involved  a  change  in  the  notochordal  structure. 
Greater  muscular  endowments  presupposed  a  firmer  internal  fulcrum 


534 

— a  less  yielding  central  axis.  On  the  other  hand,  for  tl/e  central 
axis  to  have  become  firmer  while  remaining  continuous,  would  have 
entailed  a  stiffness  incompatible  with  the  creature's  movements. 
Hence,  increasing  density  of  the  central  axis  necessarily  went  hand 
in  hand  with  its  segmentation:  for  strength,  ossification  was  re- 
quired ;  for  flexibility,  division  into  parts.  The  production  of  ver- 
tebra) resulting  thus,  there  obviously  would  arise  among  them  a 
general  likeness,  due  to  the  similarity  in  their  mechanical  conditions, 
and  more  especially  the  muscular  forces  bearing  on  them.  And  then 
observe,  lastly,  that  where,  as  in  the  head,  the  terminal  position  and 
the  less  space  for  development  of  muscles,  entailed  smaller  lateral 
bendings,  the  segmentation  would  naturally  be  less  decided,  less 
regular,  and  would  be  lost  as  we  approached  the  front  of  the 
head. 

But,  it  may  be  replied,  this  hypothesis  does  not  explain  all  the 
facts.  It  does  not  tell  us  why  a  bone  whose  function  in  a  given 
animal  requires  it  to  be  solid,  is  formed  not  of  a  single  piece,  but  by 
the  coalescence  of  several  pieces,  which  in  other  creatures  are  sepa- 
rate ;  it  does  not  account  for  the  frequent  manifestations  of  unity  of 
plan  in  defiance  of  teleological  requirements.  This  is  quite  true. 
But  it  is  not  true,  as  Professor  Owen  argues  respecting  such  cases, 
that  "  if  the  principle  of  special  adaptation  fails  to  explain  them,  and 
we  reject  the  idea  that  these  correspondences  are  manifestations  of 
some  archetypal  exemplar,  on  which  it  has  pleased  the  Creator  to 
frame  certain  of  his  living  creatures,  there  remains  only  the  alterna- 
tive that  the  organic  atoms  have  concurred  fortuitously  to  produce  such 
harmony."  This  is  not  the  only  alternative  :  there  is  another,  which 
Professor  Owen  has  overlooked.  It  is  a  perfectly  tenable  supposi- 
tion that  all  higher  vertebrate  forms  have  arisen  by  the  superposing  of 
adaptations  upon  adaptations.  Either  of  the  two  antagonist  cosmo- 
gonies consists  with  this  supposition.  If,  on  the  one  hand,  we  con- 
ceive species  to  have  resulted  from  acts  of  special  creation ;  then  it 
is  quite  a  fair  assumption  that  to  produce  a  higher  vertebrate  animal, 
the  Creator  did  not  begin  afresh,  but  took  a  lower  vertebrate  animal, 
and  so  far  modified  its  pre-existing  parts  as  to  fit  them  for  the  new 
requirements ;  in  which  case  the  original  structure  would  show  itself 
through  the  superposed  modifications.  If,  on  the  other  hand, 
we  'conceive  species  to  have  resulted  by  gradual  differentiations 
under  the  influence  of  changed  conditions;  then,  it  would  mani- 
festly follow  that  the  higher,  heterogeneous  forms,  would  bear 
traces  of  the  lower  and  more  homogeneous  forms  from  which  they 
were  evolved. 

Thus,  besides  finding  that  the  hypothesis  of  an  "  ideal  typical 
vertebra  "  is  irreconcilable  with  the  fac^s,  we  find  that  the  facts  are 
interpretable  without  gratuitous  assumptions.  The  average  com- 
munity of  form  which  vertebras  display,  is  explicable  as  resulting 


535 


fror.i  natural  causes.  And  those  typical  similarities  which  are  trace- 
able under  adaptive  modifications,  must  obviously  exist  if,  through- 
out creation  in  general,  there  has  gone  on  that  continuous  super 
poi  ing  of  modifications  upon  modifications  which  goes  on  in  every 
unfolding  organism. 


[I  might  with  propriety  have  added  to  the  foregoing  criticisms, 
the  remark  that  Professor  Owen  has  indirectly  conferred  a  great 
benefit  by  the  elaborate  investigations  he  has  made  with  the  view  of 
establishing  his  hypothesis.  He  has  himself  very  conclusively  proved 
that  the  teleological  interpretation  is  quite  irreconcilable  with  the 
facts.  In  gathering  together  evidence  in  support  of  his  own  con- 
ception of  archetypal  forms,  he  has  disclosed  adverse  evidence  which 
I  think  shows  his  conception  to  be  untenable.  The  result  is  that 
the  field  is  left  clear  for  the  hypothesis  of  Evolution  as  the  only 
tenable  one.] 


APPENDIX    C. 


[From  the  TRANSACTIONS  OP  THE  LINNEAN  SOCIETY,  VOL. 


XV.  On  Circulation  and  the  Formation  of  Wood  in  Plants.  Hi, 
HERBERT  SPENCER,  Esq.  Communicated  by  GEORGE  BUSK, 
Esq.,  F.R.S.,  Sec.  L.S. 

Read  March  1st,  1866. 

OPINIONS  respecting  the  functions  of  the  vascular  tissues  in  plants 
appear  to  make  but  little  progress  towards  agreement.  The  suppo- 
sition that  these  vessels  and  strings  of  partially-united  cells,  lined 
with  spiral,  annular,  reticulated,  or  other  frameworks,  are  carriers 
of  the  plant-juices,  is  objected  to  on  the  ground  that  they  often 
contain  air  :  as  the  presence  of  air  arrests  the  movement  of  blood 
through  arteries  and  veins,  its  presence  in  the  ducts  of  stems  and 
petioles  is  assumed  to  unfit  them  as  channels  for  sap.  On  the 
other  hand,  that  these  structures  have  a  respiratory  office,  as  some 
have  thought,  is  certainly  not  more  tenable,  since,  if  the  presence 
of  air  in  them  negatives  the  belief  that  their  function  is  to  dis- 
tribute liquid,  the  presence  of  liquid  in  them  equally  negatives  the 
belief  that  their  function  is  to  distribute  air.  Nor  can  any  better 
defence  be  made  for  the  hypothesis  which  I  find  propounded,  that 
these  parts  serve  "  to  give  strength  to  the  parenchyma."  Tubes 
with  fenestrated  and  reticulated  internal  skeletons  have,  indeed, 
some  power  of  supporting  the  tissue  through  which  they  pass  ;  but 
tubes  lined  with  spiral  threads  can  yield  extremely  little  support, 
while  tubes  lined  with  annuli,  or  spirals  alternating  with  annuli,  can 
yield  no  support  whatever.  Though  all  these  types  of  internal 
framework  are  more  or  less  efficient  for  preventing  closure  by 
lateral  pressure,  they  are  some  of  them  quite  useless  for  holding 
up  the  mass  through  which  the  vessels  pass  ;  and  the  best  of  them 
are  for  this  purpose  mechanically  inferior  to  the  simple  cylinder. 
The  same  quantity  of  matter  made  into  a  continuous  tube  would  be 
more  effective  in  giving  stiffness  to  the  cellular  tissue  around  it. 

In  the  absence  of  any  feasible  alternative,  the  hypothesis  that 
these  vessels  are  distributors  of  sap  claims  reconsideration.  The 
objections  are  not,  I  think,  so  serious  as  they  seem.  The  habitual 


537 

presence  of  air  in  the  ducts  that  traverse  wood,  can  scarcely  be 
held  anomalous  if  when  the  wood  is  formed  their  function  ceases. 
The  canals  which  ramify  through  a  Stag's  horn,  contain  air  after 
the  Stag's  horn  is  fully  developed ;  but  it  is  not  thereby  rendered 
doubtful  whether  it  is  the  function  of  arteries  to  convey  blood. 
A  gain,  that  air  should  frequently  be  found  even  in  the  vessels  of 
petioles  and  leaves,  will  not  appear  remarkable  when  we  call  to 
wind  the  conditions  to  which  a  leaf  is  subject.  Evaporation  is 
going  on  from  it.  The  thinner  liquids  pass  by  osmose  out  of  the 
vessels  into  the  tissues  containing  the  liquids  thickened  by  evapora- 
tion. And  as  the  vessels  are  thus  continually  drained,  a  draught  is 
made  upon  the  liquid  contained  in  the  stem  and  roots.  Suppose 
that  this  draught  is  unusually  great,  or  suppose  that  around  the 
roots  there  exists  no  adequate  supply  of  moisture.  A  state  of 
capillary  tension  must  result — a  tendency  of  the  liquid  to  pass  into 
the  leaves  resisted  below  by  liquid  cohesion.  Now,  had  the  vessels 
impermeable  coats,  only  their  upper  extremities  would  under  these 
conditions  be  slowly  emptied.  But  their  coats,  hi  common  with  all 
the  surrounding  tissues,  are  permeable  by  air.  Hence,  under  this 
state  of  capillary  tension,  air  will  enter  ;  and  as  the  upper  ends  of 
the  tubes,  being  both  smaller  in  diameter  and  less  porous  than  tho 
lower,  will  retain  the  liquids  with  greater  tenacity,  the  air  will 
enter  the  wider  and  more  porous  tubes  below— the  ducts  of  the 
stem  and  branches.  Thus  the  entrance  of  air  no  more  proves  that 
these  ducts  are  not  sap-carriers,  than  does  the  emptiness  of  tropical 
river-beds  in  the  dry  season  prove  that  they  are  not  channels  for 
water.  There  is,  however,  a  difficulty  which  seems  more  serious. 
It  is  said  that  air,  when  present  in  these  minute  canals,  must  be  a 
great  obstacle  to  the  movement  of  sap  through  them.  The  investi- 
gations of  Jam  in  have  shown  that  bubbles  in  a  capillary  tube  resist 
the  passage  of  liquid,  and  that  their  resistance  becomes  very  great 
when  the  bubbles  are  numerous — reaching,  in  some  experiments,  as 
much  as  three  atmospheres.  Nevertheless  the  inference  that  any 
such  resistance  is  offered  by  the  air-bubbles  in  the  vessels  of  a 
plant,  is,  I  think,  an  erroneous  one.  What  happens  in  a  capillary 
tube  having  impervious  sides,  with  which  these  experiments  were 
made,  will  by  no  means  happen  in  a  capillary  tube  having  pervious 
fides.  Any  pressure  brought  to  bear  on  the  column  of  liquid  con- 
tained in  the  porous  duct  of  a  plant,  must  quickly  cause  the  expul- 
sion of  a  contained  air-bubble  through  the  minute  openings  in  the 
coats  of  the  duct.  The  greater  molecular  mobility  of  gases  than 
liquids,  implies  that  air  will  pass  out  far  more  readily  than  sap. 
"Whilst,  therefore,  a  slight  tension  on  the  column  of  sap  will  cause 
it  to  part  and  the  air  to  enter,  a  slight  pressure  upon  it  will  force 
out  the  air  and  reunite  the  divided  parts  of  the  column. 

To  obtain  data  for  an  opinion  on  this  vexed  question,  I  have 


538 

lately  been  experimenting  on  the  absorption  of  dyes  by  plants.  So 
far  as  I  can  learn,  experiments  of  this  kind  have  most,  if  not  all  of 
them.  been  made  on  stems,  and,  as  it  would  seem  from  the  results. 
on  stems  so  far  developed  as  to  contain  all  their  characteristic 
structures.  The  first  experiments  I  made  myself  were  on  suet 
parts,  and  yielded  evidence  that  served  but  little  to  elucidate 
matters.  It  was  only  after  trying  like  experiments  with  leaves  of 
different  ages  and  different  characters,  and  with  undeveloped  axes, 
as  well  as  with  axes  of  special  kinds,  that  comprehensible  results 
were  reached ;  and  it  then  became  manifest  that  the  appearances 
presented  by  ordinary  stems  when  thus  tested,  are  in  a  great  degree 
misleading.  Let  me  briefly  indicate  the  differences. 

If  an  adult  shoot  of  a  tree  or  shrub  be  cut  off,  and  have  its  lower 
end  placed  in  an  aluined  decoction  of  logwood  or  a  dilute  solution 
of  magenta,*  the  dye  will,  in  the  course  of  a  few  hours,  ascend  to  a 
distance  varying  according  to  the  rate  of  evaporation  from  the 
leaves.  On  making  longitudinal  sections  of  the  part  traversed  by 
it,  the  dye  is  found  to  have  penetrated  extensive  tracts  of  the 
woody  tissue  ;  and  on  making  transverse  sections,  the  openings  of 
the  ducts  appear  as  empty  spaces  in  the  midst  of  a  deeply-coloured 
prosenchyma.  It  would  thus  seem  that  the  liquid  is  carried  up  the 
denser  parts  of  the  vascular  bundles ;  neglecting  the  cambium  layer, 
neglecting  the  central  pith,  and  neglecting  the  spiral  vessels  of  the 
medullary  sheath.  Apparently  the  substance  of  the  wood  has 
afforded  the  readiest  channel.  When,  however,  we  examine  these 
appearances  critically,  we  find  reasons  for  doubting  this  conclusion. 
If  a  transverse  section  of  the  lower  part,  into  which  the  dye  passed 
first  and  has  remained  longest,  be  compared  with  a  transverse  sec- 
tion of  the  part  which  the  dye  has  but  just  reached,  a  marked 
difference  is  visible.  In  the  one  case  the  whole  of  the  dense  tissue 
is  stained  ;  in  the  other  case  it  is  not.  This  uneven  distribution  of 
stain  in  the  part  which  the  dye  has  incompletely  permeated  is  not 
at  random  ;  it  admits  of  definite  description.  A  tolerably  regular 
continuous  ring  of  colour  distinguishes  the  outer  part  of  the  wood 
from  the  inner  mass,  implying  a  passage  of  liquid  up  the  elongated 
cells  next  the  cambium  layer.  And  the  inner  mass  is  coloured  more 
round  the  mouths  of  the  pitted  ducts  than  elsewhere :  the  dense 
tissue  is  darkest  close  to  the  edges  of  these  ducts ;  the  colour  fades 
away  gradually  on  receding  from  their  edges  ;  there  is  most  colour 
where  there  are  several  ducts  together ;  and  the  dense  tissue  which 

*  These  two  dyes  have  affinities  for  different  components  of  the  tissues, 
and  may  be  advantageously  used  in  different  cases.  Magenta  is  rapidly 
taken  up  by  woody  matter  and  other  secondary  deposits  ;  while  logwood 
colours  the  cell -membranes,  and  takes  but  reluctantly  to  the  substances 
seized  by  magenta.  By  trying  both  of  them  on  the  same  structure,  we  may 
guard  ourselves  against  any  error  arising  from  selective  combination. 


539 

IB  fully  dyed  for  some  space,  is  that  which  lies  between  two  or  more 
ducts.  These  are  indications  that  while  the  layer  of  pitted  cells 
next  the  cambium  has  served  as  a  channel  for  part  of  the  liquid,  the 
rest  has  ascended  the  pitted  ducts,  and  oozed  out  of  these  into  the 
prosenchyma  around.  And  this  conclusion  is  confirmed  by  the 
contrast  between  the  appearances  of  the  lowest  part  of  a  shoot 
under  different  conditions.  For  if,  instead  of  allowing  the  dye 
time  for  oozing  through  the  prosenchyma,  the  end  of  the  shoot  be 
just  dipped  into  the  dye  and  taken  out  again,  we  find,  on  making 
transverse  sections  of  the  part  into  which  the  dye  has  been  rapidly 
taken  up,  that,  though  it  has  diffused  to  some  distance  round  the 
ducts,  it  has  left  tracts  of  wood  between  the  ducts  uncoloured — a 
difference  which  would  not  exist  had  the  ascent  been  through  the 
substance  of  the  wood.  Even  still  stronger  is  the  confirmation 
obtained  by  using  one  dye  after  another.  If  a  shoot  that  has  ab- 
sorbed magenta  for  an  hour  be  placed  for  five  minutes  in  the  log- 
wood decoction,  transverse  sections  of  it  taken  at  a  short  distance 
from  its  end  show  the  mouths  of  the  ducts  surrounded  by  dark 
stains  in  the  midst  of  the  much  wider  red  stains. 

Based  on  these  comparisons  only,  the  inference  pointed  out  has 
little  weight ;  but  its  weight  is  increased  by  the  results  of  experi- 
ments on  quite  young  shoots,  and  shoots  that  develope  very  little 
wood.  The  behaviour  of  these  corresponds  perfectly  with  the  ex- 
pectation that  a  liquid  will  ascend  capillary  tubes  in  preference  to 
simple  cellular  tissue  or  tissue  not  differentiated  into  continuous  canals. 
The  vascular  bundles  of  the  medullary  'sheath  are  here  the  only 
channels  which  the  coloured  liquid  takes.  In  sections  of  the  parts 
up  to  which  the  dye  has  but  just  reached,  the  spiral,  fenestrated, 
scalariform,  or  other  vessels  contained  in  these  bundles  are  alone 
coloured  ;  and  lower  down  it  is  only  after  some  hours  that  such  an 
exudation  of  dye  takes  place  as  suffices  partially  to  colour  the  other 
substances  of  the  bundle.  Further,  it  is  to  be  noted  that  at  the 
terminations  of  shoots,  where  the  vessels  are  but  incompletely  formed 
out  of  irregularly-joined  fibrous  cells  which  still  retain  their  original 
shapes,  the  dye  runs  up  the  incipient  vessels  and  does  not  colour  in 
the  smallest  degree  the  surrounding  tissue. 

Experiments  with  leaves  bring  out  parallel  facts.  On  placing  in 
a  dye  a  petiole  of  an  adult  leaf  of  a  tree,  and  putting  it  before  the 
fire  to  accelate  evaporation,  the  dye  will  be  found  to  ascend  the 
midrib  and  veins  at  various  rates,  up  even  to  a  foot  per  hour.  At 
first  it  is  confined  to  the  vessels  ;  but  by  the  time  it  has  reached  the 
point  of  the  leaf,  it  will  commonly  be  seen  that  at  the  lower  part  it 
has  diffused  itself  into  the  sheaths  of  the  vessels.  In  a  quite  young 
leaf  from  the  same  shoot,  we  find  a  much  more  rigorous  restriction 
of  the  dyp  to  the  vessel*. "  On  making  oblique  sections  of  its  petiole, 
•iiiculb,  and  veins,  the  vessels  have  the  appearance  of  groups  of 


540 

sharply  defined  coloured  rods  imbedded  in  the  green  prosenchyrua ; 
and  this  marked  contrast  continues  with  scarcely  an  appreciable 
change  after  plenty  of  time  has  been  allowed  for  exudation. 

The  facts  thus  grouped  and  thus  contrasted  seem,  at  first  sight, 
to  imply  that  while  they  are  young  the  coats  of  these  ramifying 
canals  lined  with  spiral  or  allied  structures  are  not  readily  perme- 
able, but  that,  becoming  porous  as  they  grow  old,  they  allow  the 
liquids  they  carry  to  escape  with  increasing  facility ;  and  hence  a 
possible  interpretation  of  the  fact  that,  hi  the  older  parts,  the  stain- 
ing of  the  tissue  around  the  vessels  is  so  rapid  as  to  suggest  that  the 
dye  has  ascended  directly  through  this  tissue,  whereas  in  the  younger 
parts  the  reverse  appearance  necessitates  the  reverse  conclusion. 
But  now,  is  this  difference  determined  by  difference  of  age,  or  is  it 
otherwise  determined  ?  The  evidence  as  presented  in  ordinary  stems 
and  leaves  shows  us  that  the  parts  of  the  vascular  system  at  which 
there  is  a  rapid  escape  of  dye  are  not  simply  older  parts,  but  are 
parts  where  a  deposit  of  woody  matter  is  taking  place.  Is  it,  then, 
that  the  increasing  permeability  of  the  ducts,  instead  of  being 
directly  associated  with  their  increasing  age,  is  directly  associated 
with  the  increasing  deposit  of  dense  substance  around  them  ? 

To  get  proof  that  this  last  connexion  is  the  true  one,  we  have1 
but  to  take  a  class  of  cases  in  which  wood  is  formed  only  to  a  small 
extent.  In  such  cases  experiments  show  us  a  far  more  general  and 
continued  limitation  of  the  dye  to  the  vessels.  Ordinary  herbs  and 
vegetables,  when  contrasted  with  shrubs  and  trees,  illustrate  this  ;  as 
instance  the  petioles  of  Celery,  or  of  the  common  Dock,  and  the 
leaves  of  Cabbages  or  Turnips.  And  then  in  very  succulent  plants, 
such  as  Hryop/iyllum  calycinum,  Kalanchoe  rotundifolia,  the  various 
species  of  Crassula,  Cotyledon,  Kleinia,  and  others  of  like  habit,  the 
ducts  of  old  and  young  leaves  alike  retain  the  dye  very  persistently  : 
the  concomitant  in  these  cases  being  the  small  amount  of  prosen- 
chyma  around  the  ducts,  or  the  small  amount  of  deposit  in  it,  or 
both.  More  conclusive  yet  is  the  evidence  which  meets  us  when  we 
turn  from  very  succulent  leaves  to  very  succulent  axes.  The  tender 
young  shoots  of  Kleinia  ante-eitphorbiinn,  or  Euphorbia  Jfauritanica, 
which  for  many  inches  of  their  lengths  have  scarcely  any  ligneous 
fibres,  show  us  scarcely  any  escape  of  the  coloured  liquid  from  the 
vessels  of  the  medullary  sheath.  So,  too,  is  it  with  Stapelia 
ttu/onia,  a  plant  of  another  order,  having  soft  swollen  axes.  Ana 
then  we  have  a  repetition  of  the  like  connexion  of  facts  throughout 
tv:e  Cactacece :  the  most  succulent  showing  us  the  smallest  perme- 
ability of  the  vessels.  In  two  species  of  Jthipsalis,  in  two  species  of 
Cereus,  and  in  two  species  of  Mammillaria,  which  I  have  tried,  I 
have  found  this  so.  Mammillaria  gracilis  may  be  named  as  ex- 
emplifying the  relation  under  its  extreme  form.  Into  one  of  these 
small  spheroidal  masses,  the  dye  ascends  through  the  large  bundles 


541 

?f  spiral  or  annular  dacts,  or  cells  partially  united  into  such  ducts, 
colouring  them  deeply,  and  leaving  the  feebly-marked  sheath  of 
prosenchyma,  together  with  the  surrounding  watery  cellular  tissue, 
perfectly  uncoloured. 

The  most  conclusive  evidence,  however,  is  furnished  by  those 
Cttctacecr.  in  which  the  transition  from  succulent  to  dense  tissue 
lakes  place  variably,  according  as  local  circumstances  determine. 
Opuntia  yields  good  examples.  If  a  piece  of  it  including  one  of 
the  joints  at  which  wood  is  beginning  to  form,  be  allowed  to  absorb 
a  coloured  liquid,  the  liquid,  running  up  the  irregular  bundles  of 
vessels  and  into  many  of  their  minute  ramifications,  is  restricted  to 
these  where  they  pass  through  the  parenchyma  forming  the  mass  of 
the  stem ;  but  near  the  joints  the  hardened  tissue  around  the  vessels 
is  coloured.  In  one  of  these  fleshy  growths  we  get  clear  evidence 
that  the  escape  of  the  dye  has  no  immediate  dependence  on  the  age 
of  the  vessels,  since,  in  parts  of  the  stem  that  are  alike  in  age,  some 
of  the  vessels  retain  their  contents  while  others  do  not.  Kay,  we 
even  find  that  the  younger  vessels  are  more  pervious  than  the  older 
ones,  if  round  the  younger  ones  there  is  a  formation  of  wood. 

Thus,  then,  is  confirmed  the  inference  before  drawn,  that  in  ordi- 
nary stems  the  staining  of  the  wood  by  an  ascending  coloured  liquid 
is  due,  not  to  the  passage  of  the  coloured  liquid  up  the  substance 
of  the  wood,  but  to  the  permeability  of  its  ducts  and  such  of  its 
pitted  cells  as  are  united  into  irregular  canals.  And  the  facts 
showing  this,  at  the  same  indicate  with  tolerable  clearness  the  process 
by  which  wood  is  formed.  What  in  these  cases  is  seen  to  take  place 
with  a  dye,  may  be  fairly  presumed  to  take  place  with  sap.  Where 
the  dye  exudes  but  slowly,  we  may  infer  that  the  sap  exudes  but 
slowly  ;  and  it  is  a  fair  inference  that  where  the  dye  leaks  rapidly  out 
of  the  vessels,  the  sap  does  the  same.  Inferring,  thus,  that  where- 
ever  there  is  a  considerable  formation  of  wood  there  is  a  considerable 
escape  of  the  sap,  we  see  in  the  one  the  result  of  the  other.  The 
thickening  of  the  prosenchyma  is  proportionate  to  the  quantity  of 
nutritive  liquid  passing  into  it ;  and  this  nutritive  liquid  passes  into 
it  from  the  vessels,  ducts,  and  irregular  canals  it  surrounds. 

But  an  objection  is  made  to  such  experiments  as  the  foregoing, 
and  to  all  the  inferences  drawn  from  them.  It  is  said  that  portions 
of  plants  cut  off  and  thus  treated,  have  their  physiological  actions 
arrested,  or  so  changed  as  may  render  the  results  misleading  ;  and  it 
is  said  that  when  detached  shoots  and  leaves  have  their  cut  ends 
placed  in  solutions,  the  open  mouths  of  their  vessels  and  ducts  aro 
directly  presented  with  the  liquids  to  be  absorbed,  which  does  not 
happen  in  then-  natural  states.  Further,  making  these  objections 
look  serious,  it  is  alleged  that  when  solutions  are  absorbed  through 
the  roots,  quite  different  results  are  obtained :  the  absorbed  matters 
are  found  in  the  tissues  and  not  in  the  vessels.  Clearly,  were  the  ex- 


542 

pcriments  yielding  these  adverse  results  conducted  in  unobjectionable 
ways,  the  conclusion  implied  by  them  would  negative  the  conclusions 
above  drawn.  But  tfaese  experiments  are  no  less  objectionable  than 
those  to  which  they  are  opposed.  Such  mineral  matters  as  salts  of  iron, 
solutions  of  which  have  in  some  cases  been  supplied  to  the  roots  for 
their  absorption,  are  obviously  so  unlike  the  matters  ordinarily  absorbed, 
that  they  are  likely  to  interfere  fatally  with  the  physiological  actions. 
If  experiments  of  this  kind  are  made  by  immersing  the  roots  in  a 
dye,  there  is,  besides  the  difficulty  that  the  mineral  mordant  contained 
by  the  dye  is  injurious  to  the  plant,  the  further  difficulty  that  the 
colouring  matter,  being  seized  by  the  substances  for  which  it  has  an 
affinity,  is  left  behind  in  the  first  layers  of  root  tissues  passed  through, 
and  that  the  decolorized  water  passing  up  into  the  plant  is  not  trace- 
able. To  be  conclusive,  then,  an  experiment  on  absorption  through 
roots  must  be  made  with  some  solution  which  will  not  seriously  in- 
terfere with  the  plant's  vital  processes,  and  which  will  not  have  its 
distinctive  element  left  behind.  To  fulfil  these  requirements  I 
adopted  the  following  method.  Having  imbedded  a  well-soaked 
broad-bean  in  moist  sand,  contained  in  an  inverted  cone  of  card- 
board with  its  apex  cut  off  for  the  radicle  to  come  through — having 
placed  this  in  a  wide-mouthed  dwarf  bottle,  partly  filled  with  water, 
so  that  the  protruding  radicle  dipped  into  the  water — and  having 
waited  until  the  young  bean  had  a  shoot  some  three  or  more  inches 
high,  and  a  cluster  of  secondary  rootlets  from  an  inch  to  an  inch 
and  a-half  long — I  supplied  for  its  absorption  a  simple  decoction  of 
logwood,  which,  being  a  vegetal  matter,  was  not  likely  to  do  it  much 
harm,  and  which,  being  without  a  mordant,  would  not  leave  its  sus- 
pended colour  in  the  first  tissues  passed  through.  To  avoid  any 
possible  injury,  I  did  not  remove  the  plant  from  the  bottle,  but 
slightly  raising  the  cone  out  of  its  neck,  I  poured  away  the  water 
through  the  crevice  and  then  poured  in  the  logwood  decoction ;  so 
that  there  could  have  been  no  broken  end  or  abraded  surface  of  a 
rootlet  through  which  the  decoction  might  enter.  Being  prepared 
with  some  chloride  of  tin  as  a  mordant,  I  cut  off,  after  some  three 
hours,  one  of  the  lowest  leaves,  expecting  that  the  application  of  the 
mordant  to  the  cut  surface  would  bring  out  the  characteristic  colour 
if  the  logwood  decoction  had  risen  to  that  height.  I  got  no  re- 
action, however.  But  after  eight  hours  I  found,  on  cutting  off 
another  leaf, -that  the  vessels  of  its  petiole  were  made  visible  as  dark 
streaks  by  the.  colour  with  which  they  were  charged — a  colour  differ- 
ing, as  was  to  be  expected,  from  that  of  the  logwood  decoction, 
which  spontaneously  changes  even  by  simple  exposure.  It  was  then 
too  late  in  the  day  to  pursue  the  observations  ;  but  next  morning  the 
vessels  of  the  whole  plant,  as  far  as  the  petioles  of  its  highest  un- 
folded leaves,  were  full  of  the  colouring-matter ;  and  on  applying 
chloride  of  tin  to  the  cut  surfaces,  the  vessels  assumed  that  purplish 


648 

red  \vhieh  this  mordant  produces  when  directly  mixed  with  tho  log- 
wood decoction.  Subsequently,  when  one  of  the  cotyledons  was  cut 
open  by  Prof.  Oliver,  to  whom,  in  company  with  "Dr.  Hooker,  I 
showed  the  specimen,  we  found  that  the  whole  of  its  vascular  system 
was  filled  with  the  decoction,  which  everywhere  gave  the  characteristic 
reaction.  And  it  became  manifest  that  the  liquid  absorbed  through 
the  rootlets,  in  the  central  vessels  of  which  it  was  similarly  traceable, 
had  part  of  it  passed  directly  up  the  vessels  of  the  axis,  while  part  of 
it  had  passed  through  other  vessels  into  the  cotyledon,  out  of  which, 
no  doubt,  the  liquid  ordinarily  so  carried  returns  charged  with  a 
supply  of  the  stored  nutriment.  I  have  since  obtained  a  verification 
by  varying  the  method.  Digging  up  some  young  plants  (Marigolds 
happened  to  afford  the  best  choice)  with  large  masses  of  soil  round 
them,  placing  them  in  water,  so  as  gradually  to  detach  the  soil  with- 
out injuring  the  rootlets,  planting  them  afresh  in  a  flower-pot  full  of 
washed  sand,  and  then,  after  a  few  days,  watering  them  with  a  log- 
wood decoction,  I  found,  as  before,  that  in  less  than  twenty-four 
hours  the  colouring-matter  had  run  up  into  the  vessels  of  the  leaves. 
Though  the  reaction  produced  by  the  mordant  was  not  so  strong  as 
before,  it  was  marked  enough  to  be  quite  unquestionable. 

As  these  experiments  were  so  conducted  that  there  was  no  access 
to  the  vessels  except  through  the  natural  channels,  and  as  the  vital 
actions  of  the  plants  were  so  little  interfered  with  that  at  the  end  of 
twenty-four  hours  they  showed  no  traces  of  disturbance,  I  think  the 
results  must  be  held  conclusive. 

Taking  it,  then,  as  a  fact  that  in  plants  possessing  them  the  vessels 
and  ducts  are  the  channels  through  which  sap  is  distributed,  we  come 
now  to  the  further  question — What  determines  the  varying  permea- 
bility of  the  walls  of  the  vessels  and  ducts,  and  the  consequent  vary- 
ing formation  of  wood  ?  To  this  question  I  believe  the  true  reply  is — 
The  exposure  of  the  parts  to  intermittent  mechanical  strains,  actual 
or  potential,  or  both.  By  actual  strains  I  of  course  mean  those 
which  the  plant  experiences  in  the  course  of  its  individual  life.  By 
potential  strains  I  mean  those  which  the  form,  attitude,  and  circum- 
stances common  to  its  kind  involve,  and  which  its  inherited  structure 
is  adapted  to  meet.  In  plants  with  stems,  petioles,  and  leaves, 
having  tolerably  constant  attitudes,  the  increasing  porosity  of  the 
tubes  and  consequent  deposit  of  dense  tissue  takes  place  in  anticipa- 
tion of  the  strains  to  which  the  parts  of  the  individual  are  liable,  but 
lakes  place  at  parts  which  have  been  habitually  subject  to  such 
strains  in  ancestral  individuals.  But  though  in  such  plants  the 
tendency  to  repeat  that  distribution  of  dense  tissue  caused  by 
mechanical  actions  on  past  generations,  goes  on  irrespective  of  the 
mechanical  actions  to  which  the  developing  individual  is  subject, 
these  direct  actions,  while  they  greatly  aid  the  assumption  of  the 
typical  structure,  are  the  sole  causes  of  those  deviations  in  the  rela- 


644 

live  thickenings  of  parts  which  distinguish  the  individual  from  others 
of  its  kind.  And  then,  in  certain  irregularly  growing  plants,  such  as 
Cactuses  and  Euphorbias,  where  the  strains  fall  on  parts  that  do 
not  correspond  in  successive  individuals,  we  distinctly  trace  a  direct 
relat;on  between  the  degrees  of  strain  anJ  the  rates  of  these  changes 
which  result  in  dense  tissue.  I  will  not  occupy  space  in  detailing 
the  evidence  of  this  relation,  which  is  conspicuous  in  the  orders 
named,  but  will  pass  to  the  question — What  are  the  physical  processes 
by  which  intermittent  mechanical  strains  produce  this  deposit  of 
resistant  substance  at  places  where  it  is  needed  to  meet  the  strains  ? 
We  have  not  to  seek  far  for  an  answer.  If  a  trunk,  a  bough,  a 
shoot,  or  a  petiole,  is  bent  by  a  gust  of  wind,  the  substance  of  its 
convex  side  is  subject  to  longitudinal  tension :  the  substance  of  its 
concave  side  being  at  the  same  time  compressed.  This  is  the 
primary  mechanical  effect.  There  is,  however,  a  secondary  mechani- 
cal effect,  which  here  chiefly  concerns  us.  That  bend  by  which  the 
tissues  of  the  convex  side  are  stretched,  also  produces  lateral  com- 
pression of  them.  Buttoning  on  a  tight  glove  and  then  closing  the 
hand,  will  make  this  necessity  clear  :  the  leather,  while  it  is  strained 
along  the  backs  of  the  fingers,  presses  with  considerable  force  on  the 
knuckles.  It  is  demonstrable  that  the  tensions  of  the  outer  layei  of 
a  mass  made  convex  by  bending,  must,  by  composition  of  forces, 
produce  at  every  point  a  resultant  at  right  angles  to  the  layer  be- 
neath it ;  that,  similarly,  the  joint  tensions  of  these  two  layers  must 
throw  a  pressure  on  the  next  deeper  layer ;  and  so  on.  Hence,  i'f 
at  some  little  distance  beneath  the  surface  of  a  stem,  twig,  or  leaf- 
stalk, there  exist  longitudinal  tubes,  these  tubes  must  be  squeezed 
each  time  the  side  of  the  branch  they  are  placed  on  becomes  convex. 
Modifying  the  illustration  just  drawn  from  the  clenched  hand  will 
make  this  clear.  When,  on  forcibly  grasping  something,  the  skin  is 
drawn  tightly  over  the  back  of  the  hand,  the  whitening  of  the 
knuckles  shows  how  the  blood  is  expelled  from  the  vessels  below  the 
surface  by  the  pressure  of  the  tightened  skin.  If,  then,  the  sap- 
vessels  must  be  thus  compressed,  what  will  happen  to  the  liquid  they 
contain  ?  It  will  move  away  along  the  lines  of  least  resistance. 
Part,  and  probably  the  greater  part,  will  escape  lengthways  from  the 
place  of  greatest  pressure :  some  of  it  being  expelled  downwards, 
and  some  of  it  upwards.  But,  at  the  same  time,  part  of  it  will  be 
likely  to  ooze  through  the  walls  of  the  tubes.  If  these  walls  are  so 
perfect  as  to  permit  the  passage  of  liquid  only  by  osmose,  it  may 
still  be  inferred  that  the  osmose  will  increase  under  pressure ;  and 
probably,  under  recurrent  pressure,  the  places  at  which  the  osmotic 
current  passes  most  readily  will  become  more  and  more  permeable, 
until  they  eventually  form  pores.  At  any  rate  it  is  manifest  that 
where  pores  and  slits  exist,  whether  thus  formed  or  formed  in  any 
other  way,  the  escape  of  sap  into  the  adjacent  tissue  at  each  bond 


645 

wiil  become  easy  and  rapid.  What  further  must  happen  ?  "When 
the  branch  or  shoot  recoils,  the  vessels  on  the  .side  that  was  convex, 
oeiug  relieved  from  pressure,  will  tend  to  resume  their  previous 
diameters ;  and  will  be  helped  to  do  this  by  the  elasticity  of  the  sur- 
rounding tissue,  as  well  as  by  those  spiral,  annular,  and  allied  struc- 
tures which  they  contain.  But  this  resumption  of  their  previous 
diameters  must  cause  an  immediate  rush  of  sap  back  into  them. 
Whence  will  it  come?  Not  to  any  considerable  extent  from  the  sur- 
rounding tissues  into  which  part  of  it  has  been  squeezed,  seeing  that 
the  resistance  to  tha  return  of  liquid  through  small  pores  will  be 
greater  than  the  resistance  to  its  return  along  the  vessels  themselves. 
Manifestly  the  sap  which  was  thrust  up  and  down  the  vessels  from 
the  place  of  compression  will  return — the  quantities  returning  from 
above  and  from  below  varying,  as  we  shall  hereafter  see,  according 
to  circumstances.  But  this  is  not  all.  From  some  side  a  greater 
quantity  must  come  back  than  was  sent  away  ;  for  the  amount  that 
has  escaped  out  of  the  tube  into  the  prosenchyma  has  to  be  replaced. 
Thus  during  the  time  when  the  side  of  the  branch  or  twig  becomes 
concave,  more  sap  returns  from  above  or  below  than  was  expelled 
upwards  or  downwards  during  the  previous  compression.  The  re- 
filled vessels,  when  the  next  bend  renders  their  side  convex,  again 
have  part  of  their  contents  forced  through  their  parietes,  and  are 
again  refilled  in  the  same  way.  There  is  thus  set  up  a  draught  of  sap 
to  the  place  where  these  intermittent  strains  are  going  on,  an  exuda- 
tion proportionate  to  the  frequency  and  intensity  of  the  strains, 
and  a  proportionate  nutrition  or  thickening  of  the  wood- 
cells,  fitting  them  to  resist  the  strains.  A  rude  idea  of 
this  action  may  be  obtained  by  grasping  in  one  hand  a  damp 
sponge,  having  its  lower  end  in  water,  while  holding  a  piece  of 
blotting-paper  in  contact  with  its  upper  end,  and  then  giving  the 
Bponge  repeated  squeezes.  At  each  squeeze  some  of  the  water  will 
be  sent  into  the  blotting-paper ;  at  each  relaxation  the  sponge  will 
refill  from  below,  to  give  another  portion  of  its  contents  to  tlw 
blotting  paper  when  again  squeezed. 

But  how  does  this  explanation  apply  to  roots  ?  If  the  formation 
of  wood  is  due  to  intermittent  transverse  strains,  such  as  are  pro- 
duced in  the  aerial  parts  of  upright  plants  by  the  wind,  how  does  it 
happen  that  woody  matter  is  deposited  in  roots,  where  there  are  no 
lateral  oscillations,  no  transverse  strains?  The  answer  is,  that 
longitudinal  strains  also  are  capable  of  causing  the  effects  described. 
It  is  true  that  perfectly  straight  fibres  united  into  a  bundle  and  pulled 
lengthways  would  not  exert  on  one  another  any  lateral  pressure,  and 
would  not  laterally  compress  any  similarly-straight  canals  running 
ilong  with  them.  But  if  the  fibres  united  into  a  bundle  are  variously 
bent  or  twisted,  they  cannot  be  longitudinally  strained  without  com- 
pressing one  another  and  structures  imbedded  in  them.  It  needs 


546 

but  to  watch  a  wet  rope  drawn  tight  by  a  capstan,  to  see  that  an 
action  like  that  which  squeezes  the  water  out  of  its  strands,  will 
squeeze  the  sap  out  of  the  vessels  of  a  root  into  the  surrounding 
tissue,  as  often  as  the  root  is  pulled  by  the  swaying  of  the  plant  it 
belongs  to.  Here,  too,  as  before,  the  vessels  will  refill  when  the 
pull  intermits ;  and  so,  in  the  roots  as  hi  the  branches,  this  rude 
pumping  process  will  produce  a  growth  of  hard  tissue  proportionate 
to  the  stress  to  be  borne. 

These  conclusions  are  supported  by  the  evidence  which  exceptional 
cases  supply.  If  intermittent  mechanical  strains  thus  cause  the  for- 
mation of  wood  where  wood  is  found,  then  where  it  is  not  found, 
there  should  be  an  absence  of  intermittent  mechanical  strains.  There 
is  such  an  absence.  Vascular  plants  characterized  by  little  or  no 
deposit  of  dense  substance,  are  those  having  vessels  so  conditioned 
that  no  considerable  pressures  are  borne  by  them.  The  more 
succulent  a  petiole  or  leaf  becomes,  the  more  do  the  effects  of  trans- 
verse strains  fall  on  its  outer  layers  of  cells.  Its  mechanical  support 
is  chiefly  derived  from  the  ability  of  these  minute  vesicles,  full  of 
liquid,  to  resist  bursting  and  tearing  under  the  compressions  and 
tensions  they  are  exposed  to.  And  just  as  fast  as  this  change  from 
a  thin  leaf  or  foot-stalk  to  a  thick  one  entails  increasing  stress  on  the 
superficial  tissue,  so  fast  does  it  diminish  the  stress  on  the  internally- 
seated  vascular  tissue.  The  succulent  leaf  cannot  be  swayed  about 
by  the  wind  as  much  as  an  ordinary  leaf ;  and  such  small  bends  as 
can  be  given  to  it  and  its  foot-stalk  are  prevented  from  affecting  hi 
any  considerable  degree  the  tubes  running  through  its  interior. 
Hence  the  retentiveness  of  the  vessels  hi  these  fleshy  leaves,  as  shown 
by  the  small  exudation  of  dye ;  and  hence  the  small  thickening  of 
their  surrounding  prosenchyma  by  woody  deposit.  Still  more  con- 
spicuously is  this  connexion  of  facts  shown  when,  from  the  soft  thick 
leaves  before  named  and  such  others  as  those  of  Echeveria,  Rochea, 
Pereskia,  we  turn  to  the  thick  leaves  that  have  strong  exo-skeletons. 
Gasteria  serves  as  an  illustration.  The  leathery  or  horny  skin  here 
evidently  bears  the  entire  weight  of  the  leaf,  and  is  so  stiff  as  to  pre- 
vent any  oscillation.  Here,  then,  the  vessels  running  inside  are  pro- 
tected from  all  mechanical  stress ;  and  accordingly  we  find  that  the 
cells  surrounding  them  are  not  appreciably  thickened. 

Equally  clear,  and  more  striking  because  more  obviously  excep- 
tional, is  the  evidence  given  by  succulent  stems  which  are  leafless. 
Stapelia  Buffonia,  having  soft  procumbent  axes  not  liable  to  be  bent 
backwards  and  forwards  in  any  considerable  degree  by  the  wind, 
li9S,  ramifying  through  its  tissue,  vessels  that  allow  but  an  extremely 
slow  escape  of  dye  and  have  unthickened  sheaths.  Such  ol  the 
Euphorbias  as  have  acquired  the  fleshy  character  while  retaining  the 
arborescent  growth,  like  Euphorbia  Canariensis,  teach  us  the  same 
truth  in  another  way.  In  them  the  formation  of  wood  around  tho 


547 

ressels  is  inccnspicuous  where  the  intermittent  strains  are  but  slight ; 
hut  it  is  conspicuous  at  those  joints  on  which  lateral  oscillations  of 
the  attached  branches  throw  great  extensions  and  compressions  of 
tissue.  Throughout  the  Cactacece  we  find  varied  examples  of  the 
alleged  relation.  Mammillaria  furnishes  a  very  marked  one.  The 
substance  of  one  of  these  globular  masses,  resting  on  the  ground, 
admits  of  no  bending  from  side  to  side  ;  and  accordingly  its  large 
bundles  of  spiral  and  annular  vessels,  or  partially-united  cells,  have 
i  ery  feebly-marked  sheaths  not  at  all  thickened.  In  such  types  as 
Cereus  and  Opuntia  we  see,  as  in  the  Euphorbias,  that  where  little 
stress  falls  on  the  vessels,  little  deposit  takes  place  around  them  ; 
while  there  is  much  deposit  where  there  is  much  stress.  Here  let  me 
add  a  confirmation  obtained  since  writing  the  above.  After  observ- 
ing among  the  Cactuses  the  very  manifest  relation  between  strain 
and  the  formation  of  wood,  I  inquired  of  Mr.  Croucher,  the  intelli- 
gent foreman  of  the  Cactus-house  at  Kew,  whether  he  found  this 
relation  a  constant  one.  He  replied  that  he  did,  and  that  he  had 
frequently  tested  it  by  artificially  subjecting  parts  of  them  to  strains. 
Neglecting  at  the  tune  to  inquire  how  he  had  done  this,  it  afterwards 
occurred  to  me  that  if  he  had  so  done  it  as  to  cause  constant  strains, 
the  observed  result  would  not  tell  in  favour  of  the  foregoing  inter- 
pretation. Subsequently,  however,  I  learned  that  he  had  produced 
the  strains  by  placing  the  plants  in  inclined  attitudes — a  method 
which,  by  permitting  oscillations  of  the  strained  joints,  allowed  the 
strains  to  intermit.  And  then,  making  the  proof  conclusive,  Mr. 
Croucher  volunteered  the  statement  that  where  he  had  produced 
constant  strains  by  tying,  no  formation  of  wood  took  place. 

Aberrant  growths  of  another  class  display  the  same  relations  of 
phenomena.  Take  first  the  underground  stems,  such  as  the  Potato 
and  the  Artichoke.  The  vessels  which  run  through  these,  slowly 
take  up  the  dye  without  letting  it  pass  to  any  considerable  extent 
into  the  surrounding  tissues.*  Only  after  an  interval  of  many  hours 
does  the  prosenchyma  become  stained  in  some  places.  Here,  as 
before,  an  absence  of  rapid  exudation  accompanies  an  absence  of 
woody  deposit ;  and  both  these  go  along  with  the  absence  of  inter- 
mittent strains.  Take  again  the  fleshy  roots.  The  Turnip,  the 
Carrot,  and  the  Beetroot,  have  vessels  that  retain  very  persistently 
the  coloured  liquids  they  take  up.  And  differing  in  this,  as  these 
roots  do,  from  ordinary  roots,  we  see  that  they  also  differ  from  them 
in  not  being  woody,  and  in  not  being  appreciably  subject  to  the 

*  Those  who  repeat  these  experiments  must  be  prepared  for  great  irregu- 
larities in  the  rates  of  absorption.  Succulent  structures  in  general  absorb 
much  moi-e  slowly  than  others,  and  sometimes  will  scarcely  take  up  the  dye 
at  all.  The  differences  between  different  structures,  and  the  same  structure 
at  different  times,  probably  depend  on  the  degrees  in  which  the  tissues  ara 
charged  with  liquid  and  the  rates  at  which  they  are  losing  it  by  evaporation, 


548 

usual  mechanical  actions.  In  these  cases,  as  in  the  others,  parts 
that  ordinarily  become  dense,  deviate  from  this  typical  character 
when  they  are  not  exposed  to  those  forces  which  produce  dense 
'tissue  by  increasing  the  extravasation  of  sap. 

To  complete  the  proof  that  such  a  relation  exists,  let  me  add  the 
results  of  some  experiments  on  equal  and  similarly-developed  parts, 
kept  respectively  at  rest  and  in  motion.  I  have  tested  the  effects  on 
large  petioles,  on  herbaceous  shoots,  and  on  woody  shoots.  If  two 
each  petioles  as  those  of  Rhubarb,  with  their  leaves  attached,  have 
their  cut  ends  inserted  in  bottles  of  dye,  and  the  one  be  bent  back- 
wards and  forwards  while  the  other  remains  motionless,  there  arises, 
after  the  lapse  of  an  hour,  scarcely  any  difference  in  the  states  of 
their  vessels  :  a  certain  proportion  of  these  are  in  both  cases  charged 
with  the  dye,  and  little  exudation  has  been  produced  by  the  motion. 
Here,  however,  it  is  to  be  observed  that  the  causes  of  exudation  are 
scarcely  operative ;  the  vascular  bundles  are  distributed  all  through 
the  mass  of  the  petiole,  which  is  formed  of  soft  watery  tissue ;  and 
they  are,  therefore,  not  so  circumstanced  as  to  be  effectually  com- 
pressed by  the  bends.  In  herbaceous  stems,  such  as  those  of  the 
Jerusalem  Artichoke  and  of  the  Foxglove,  an  effect  scarcely  more 
decided  is  produced  ;  and  here,  too,  when  we  seek  a  reason,  we  find 
it  in  the  non-fulfilment  of  the  mechanical  conditions ;  for  the  vascular 
bundles  are  not  so  seated  between  a  tough  layer  of  bark  and  a  solid 
core  as  to  be  compressed  at  each  bend.  When,  however,  we  come 
to  experiment  upon  woody  shoots,  we  meet  with  conspicuous  effects, 
though  by  no  means  uniformly.  In  some  cases  oscillations  produce 
immense  amounts  of  exudation — parallel  transverse  sections  of  the 
compared  shoots  showing  that  where,  in  the  one  that  has  been  at 
rest,  there  are  spots  of  colour  round  but  a  few  pitted  ducts,  in  the 
one  that  has  been  kept  in  motion  the  substance  of  the  wood  is  soaked 
almost  uniformly  through  with  dye.  In  other  cases,  especially  where 
there  is  much  undifferentiated  tissue  remaining,  the  exudation  is  not 
very  marked.  The  difference  appears  to  'depend  on  the  quantity  of 
liquid  contained  in  the  shoot.  If  its  substance  is  relatively  dry,  the 
exudation  is  great ;  but  it  is  comparatively  small  if  all  the  tissues  are 
fully  charged  with  sap.  This  contrast  of  results  is  one  which  con 
templation  of  the  mechanical  actions  will  lead  us  to  expect. 

And  now,  with  these  facts  to  aid  our  interpretation,  let  us  return 
to  ordinary  stems.  If  the  upper  end  of  a  growing  shoot,  the  pro?en- 
chyma  of  which  is  but  little  thickened,  be  allowed  to  imbibe  the  dye, 
the  vessels  of  its  medullary  sheath  alone  become  charged  ;  and  from 
them  there  takes  place  but  a  slow  ooziiig.  If  a  like  experiment  be 
tried  with  a  lower  part  of  the  shoot,  where  the  wood  in  course  of 
formation  has  its  inner  boundary  marked  but  not  its  outer  boundary, 
we  find  that  the  pitted  ducts,  and  more  especially  the  inner  ones, 
come  into  play.  And  then  lower  still,  where  the  wood  has  its  peri- 


649 

phcry  defined  and  its  histological  characters  decided,  the  appearances 
show  that  the  tissue  forming  its  onter  surface  begins  to  take  a  lead- 
ing part  in  the  transmission  of  liquid.  What  now  is  the  explanation 
of  these  changes,  mechanically  considered  ?  In  the  young  soft  part 
of  the  shoot,  as  in  all  normal  and  abnormal  growths  that  have  not 
formed  wood,  the  channels  for  the  passage  of  sap  are  the  spiral, 
nnnular,  fenestrated,  or  reticulated  vessels.  These  vessels,  here  in- 
cluded in  the  bundles  of  the  medullary  sheath,  are,  in  common  with 
the  tissues  around  them,  subject,  by  the  bendings  of  the  shoot,  to 
slight  intermittent  compressions,  and,  especially  the  outermost  of 
them,  are  thus  forced  to  give  the  prosenchyma  an  extra  supply  of 
nutritive  liquid.  The  thickening  of  the  prosenchyma,  spreading 
laterally  as  well  as  outwards  from  each  bundle  of  the  medullary 
sheath,  goes  on  until  it  meets  the  thickenings  that  spread  from  the 
other  bundles  ;  and  there  is  so  formed  an  irregular  cylinder  of  har- 
dened tissue,  surrounding  the  medulla  and  the  vascular  bundles  of 
its  sheath.  As  soon  as  this  happens,  these  vascular  bundles  become, 
to  a  considerable  extent,  shielded  from  the  effects  of  transverse 
strains,  since  the  tensions  and  compressions  chiefly  fall  on  the  de- 
veloping wood  outside  of  them.  Clearly,  too,  the  greatest  stress 
must  be  felt  by  the  outer  layer  of  the  developing  wood  :  being  fur 
ther  removed  from  the  neutral  axis,  it  must  be  subject  to  severer 
strains  at  each  bend  ;  and  lying  between  the  bark  and  the  layer  of 
wood  first  formed,  it  must  be  most  exposed  to  lateral  compressions. 
Among  the  elongated  'cells  of  this  outer  layer,  some  unite  to  form 
the  pitted  ducts.  Being,  as  we  see,  better  circumstanced  mechani- 
cally, they  become  greater  carriers  of  sap  than  the  original  vessels, 
and,  in  consequence  of"  this,  as  well  as  in  consequence  of  their  rela- 
tive proximity,  become  the  sources  of  nutrition  to  the  still  more  ex- 
ternal layers  of  wood-cells.  The  same  causes  and  the  same  effects 
hold  with  each  new  indurated  coat  deposited  round  the  previously 
indurated  coats. 

This  description  may  be'  thought  to  go  far  towards  justifying  the 


current  views  respecting  the  course  taken  by  the  sap.  But  the 
justification  is  more  apparent  than  real.  In  the  first  place,  the  im- 
plication here  is  that  the  sap-carrying  function  is  at  first  discharged 
entirely  by  the  vessels  of  the  medullary  sheath,  and  that  they  cease 
to  discharge  this  function  only  as  fast  as  they  are  relatively  incapaci- 
tated by  their  mechanical  circumstances.  And  the  second  implica- 
tion is,  that  it  is  not  the  wood  itself,  but  the  more  or  less  continuous 
canals  formed  in  it,  which  are  the  subsequent  sap-distributors.  This, 
though  readily  made  clear  by  microscopic  examination  of  the  large 
pitted  ducts  in  a  partially  lignified  shoot  that  has  absorbed  the  dye, 
is  less  manifestly  true  of  the  peripheral  layer  of  sap-carrying  tissue 
finally  forme-d..  But  it  is  reallv  true  here.  For  this  layer,  though 
nominally  a  layer  of  wood,  is  practically  a  laver  of  inosculating 


550 

vessels.  It  is  formed  out  of  irregular  lines  and  networks  of  elon- 
gated pitted  cells,  obliquely  united  by  their  ends.  Examination  of 
them  after  absorption  of  a  dye,  shows  that  it  is  only  along  the  con- 
tinuous channels  they  unite  to  form  that  the  current  has  passed. 
But  the  essentially  vascular  character  of  this  outer  and  latest-formed 
layer  of  the  alburnum  is  best  seen  in  the  fact  that  the  vascular  sys- 
tems of  new  axes  take  their  rise  from  it,  and  form  with  it  continuous 
canals.  If  a  shoot  of  last  year  in  which  growth  is  recommencing,  be 
cut  lengthways  after  it  has  imbibed  a  dye,  clear  proof  is  obtained 
that  the  passage  of  the  dye  into  a  lateral  bud  takes  place  from  this 
outermost  layer  of  pitted  cells,  and  that  the  channels  taken  by  the 
dye  through  the  new  tissue  are  composed  of  cells  that  pass  through 
modified  forms  into  the  spiral  vessels  of  the  new  medullary  sheath. 
This  transition  may  be  still  more  clearly  traced  in  a  terminal  bud 
that  continues  the  line  of  last  year's  shoot.  A  longitudinal  section 
of  this  shows  that  the  vessels  of  the  new  medullary  sheath  do  not 
obtain  their  sap  from  the  vessels  of  last  year's  sheath  (which,  as 
shown  by  the  non-absorption  of  dye,  have  become  inactive),  but  that 
their  supplies  are  obtained  from  those  inosculating  canals  formed  out 
of  last  year's  outermost  layer  of  prosenchyma,  and  that  between  the 
component  cells  of  this  and  those  of  the  new  vascular  system  there 
are  all  gradations  of  structure.* 

*  It  may  be  added  here  that,  on  considering  the  mechanical  actions  that 
must  go  on,  we  are  enabled  in  somemeasure  to  understand  both  ho\v  such  inos- 
culating channels  are  initiated,  and  how  the  structures  of  their  component 
cells  are  explicable.  What  rrust  happen  to  one  of  these  elongated  prosen- 
ehyma-cel.s  if,  in  the  course  of  its  development,  it  is  subject  to  intermittent 
compressions  ?  Its  squeezed-out  liquid  while  partially  escaping  laterally, 
will  more  largely  escape  upwards  and  downwards  ;  and  while  repeated 
lateral  escape  will  tend  to  form  lateral  channels  communicating  with 
laterally-adjacent  cells,  repeated  longitudinal  escape  will  tend  to  form 
channels  communicating  with  longitudinally-adjacent  cells  —  so  pro- 
ducing continuous  though  irregular  longitudinal  canals.  Meanwhile 
each  cell  into  and  out  of  which  the  nutritive  liquid  is  from  time 
to  time  squeezed  through  small  openings  in  its  walls,  cannot  thicken 
internally  in  an  even  manner  :  deposition  will  be  interfered  with  by 
the  .passage  of  the  currents  through  the  pores.  The  rush  to  or  from  each 
pore  will  tend  to  maintain  a  funnel-shaped  depression  in  the  deposit  around  ; 
and  the  opening  from  cell  to  cell  will  so  acquire  just  that  shape  which  the 
microscope  shows  up— two  hollow  cones  with  their  apices  meeting  at  the 
point  where  the  cell -membranes  are  in  contact.  Moreover,  as  confirming 
this  interpretation,  it  may  be  remarked  that  we  are  thus  supplied  with  a 
reason  for  the  differences  of  shape  between  these  passages  from  one  pitted 
cell  to  another,  and  the  analogous  passages  that  exist  between  cells  other- 
wise formed  and  otherwise  conditioned.  In  the  cells  of  the  medulla,  and 
others  which  are  but  little  exposed  to  compression,  the  passages  are  seve- 
rally formed  more  like  a  tube  with  two  trumpet-mouths,  one  in  each  cell. 
This  is  just  the  form  which  might  be  expected  where  the  nutritive  fluid 
passes  from  cell  to  cell  in  moderate  currents,  and  not  by  the  violent  rushes 
caused  by  intermittent  pressures.  Of  course  it  is  not  meant  that  in  each 


551 

It  is  not  the  aim  of  the  foregoing  reasoning  to  show  that  mechani- 
cal actions  are  the  sole  causes  of  the  formation  of  dense  tissue  in 
plants.  Dense  tissue  is  in  many  cases  formed  where  no  such  causes 
have  come  into  play— as,  for  example,  in  thorns  and  in  the  shells  of 
nuts.  Here  the  natural  selection  of  variations  can  alone  have  ope- 
rated. It  is  manifest,  too,  that  even  those  supporting  structures  tlie 
building  up  of  which  is  above  ascribed  to  intermittent  strains,  may, 
ill  the  individual  plant  of  a  species  that  ordinarily  has  them,  be  de- 
veloped to  a  great  extent  when  intermittent  strains  are  prevented. 
"We  see  this  in  trees  that  are  artificially  supported  by  nailing  to 
walls  ;  and  we  also  see  a  kindred  fact  in  natural  climbers.  Though 
in  these  cases  the  formation  of  wood  is  obviously  less  than  it  would 
bo  were  the  stem  and  branches  habitually  moved  about  by  the  wind,  it 
nevertheless  goes  on.  Clearly  the  tendency  of  the  plant  to  repeat  tho 
structure  of  its  type  (in  the  one  case  the  structure  of  its  species,  and  in 
the  other  case  that  of  the  order  from  which  it  has  diverged  in  becom- 
ing a  climber)  is  here  almost  the  sole  cause  of  wood-formation.  But 
though  in  plants  so  circumstanced  intermittent  mechanical  strains  have 
little  or  no  direct  share,  it  may  still  be  true,  and  I  believe  is  true,  that 
intermittent  mechanical  strains  are  the  original  cause ;  for,  as  before 
hinted,  the  typical  structure  which  the  individual  thus  repeats  irre- 
spective of  its  own  conditions,  is  interpretable  as  a  typical  structure 
that  is  itself  the  product  of  these  actions  and  reactions  between  the 
plant  and  its  environment.  Grant  the  inheritance  of  functionally- 
produced  modifications ;  grant  that  natural  selection  will  always  co- 
operate in  such  way  as  to  favour  those  individuals  and  families  in 
which  functionally-produced  modifications  have  progressed  most  ad- 
vantageously ;  and  it  will  follow  that  this  mechanically-caused  forma- 
tion of  dense  substance,  accumulating  from  generation  to  generation 
by  the  survival  of  the  fittest,  will  result  in  an  organic  habit  of  form- 
ing dense  tissue  at  the  required  places.  The  deposit  arising  from 
exudation  at  the  places  of  greatest  strain,  recurring  from  generation 
to  generation  at  the  same  places,  will  come  to  be  reproduced  in  an- 
ticipation of  strain,  and  will  continue  to  be  reproduced  for  a  long 
time  after  a  changed  habit  of  the  species  prevents  the  strain — even- 
tually, however,  decreasing,  both  through  functional  inactivity  and 
natural  selection,  to  the  point  at  which  it  is  in  equilibrium  with  tho 
requirement. 

individual  cell  these  structures  are  determined  bv  these  mechanical  actions. 
The  facts  clearly  negative  any  such  conclusion,  snowing  us,  as  they  in  many 
cases  do,  that  these  structures  are  assumed  in  advance  of  these  mechanical 
actions.  The  implication  is,  that  such  mechanical  actions  initiated  modifi- 
cations that  have,  with  the  aid  of  natural  selection,  been  accumulated  from 
generation  to  generation  ;  until,  in  conformity  with  ordinary  embryological 
laws,  the  cells  of  the  parts  exposed  to  such  actions  assume  these  special 
structures  irrespective  of  the  actions— the  actions,  however,  still  3erving  to 
aid  and  complete  the  assumption  of  the  inherited  type. 
Voi.  IL  2t 


562 

Another  side  of  the  general  question  may  now  be  considered.  We 
have  seen  how,  by  intermittent  pressures  on  capillary  vessels  and 
ducts  and  inosculating  canals,  there  must  be  produced  a  draught  of 
sap  towards  the  point  of  compression  to  replace  the  sap  squeezed  out. 
But  we  have  still  to  inquire  what  will  be  the  effect  on  the  distribu- 
tion of  sap  throughout  the  plant  as  a  whole.  It  was  concluded  that 
out  of  the  compressed  vessels  the  greater  part  of  the  liquid  would 
escape  longitudinally — the  longitudinal  resistance  to  movement  being 
least.  In  every  case  the  probabilities  are  infinity  to  one  against  the 
resistances  being  equal  upwards  and  downwards.  Always,  then, 
more  sap  will  be  expelled  in  one  direction  than  in  the  other.  But  in 
whichever  direction  least  sap  is  expelled,  from  that  same  direction 
most  sap  will  return  when  the  vessels  are  relieved  from  pressure — the 
force  which  is  powerful  in  arresting  the  back  current  in  that  direction 
being  the  same  force  which  is  powerful  in  producing  a  forward  cur- 
rent. Ordinarily,  the  more  abundant  supply  of  liquid  being  from  below, 
there  will  result  an  upward  current.  At  each  bend  a  portion  of  the  con- 
tents will  be  squeezed  out  through  the  sides  of  the  vessels — a  portion 
will  be  squeezed  downwards,  reversing  the  current  ascending  from  the 
roots,  but  soon  stopped  by  its  resistance  ;  while  a  larger  portion  will 
be  squeezed  upwards  towards  the  extremities  of  the  vessels,  where 
consumption  and  loss  are  most  rapid.  At  each  recoil  the  vessels  will 
be  replenished,  chiefly  by  the  repressed  upward  current ;  and  at  the 
next  bend  more  of  it  will  be  thrust  onwards  than  backwards.  Hence 
we  have  everywhere  in  action  a  kind  of  rude  force-pump,  worked  by 
the  wind  ;  and  we  see  how  sap  may  thus  be  raised  to  a  height  far 
beyond  that  to  which  it  could  be  raised  by  capillary  action,  aided  by 
osmose  and  evaporation. 

Thus  far,  however,  the  argument  proceeds  on  the  asumption  that 
there  is  liquid  enough  to  replenish  every  time  the  vessels  subject 
to  this  process.  But  suppose  the  supply  fails — suppose  the  roots 
have  exhausted  the  surrounding  stock  of  moisture.  Evidently  the 
vessels  thus  repeatedly  having  their  contents  squeezed  out  into  the 
surrounding  tissue,  cannot  go  on  refilling  themselves  from  other 
vessels  without  tending  to  empty  the  vascular  system.  On  the  one 
hand,  evaporation  from  the  leaves  causing  a  draught  on  the  capillary 
tubes  that  end  in  them,  continually  generates  a  capillary  tension  up- 
wards ;  while,  on  the  other  hand,  the  vessels  below,  expanding  after 
their  sap  has  been  squeezed  out,  produce  a  tension  both  upwards 
and  downwards  towards  the  point  of  loss.  Were  the  limiting  mem- 
branes of  the  vessels  impermeable,  the  movement  of  sap  would,  under 
these  conditions,  soon  be  arrested.  But  these  membranes  are  perme- 
able ;  and  the  surrounding  tissues  readily  permit  the  passage  of  air. 
This  state  of  tension,  then,  will  cause  an  entrance  of  air  into  the  tubes  ; 
the  columns  of  liquid  they  contain  will  be  interrupted  by  bubbles. 
It  seems,  indeed,  not  improbable  that  this  entrance  of  air  may  take 


553 

place  even  when  there  is  a  good  supply  of  liquid,  if  the  mechanical 
strains  are  so  violent  and  the  exudation  so  rapid  that  the  currents 
cannot  refil  the  half-emptied  vessels  with  sufficient  rapidity.  And  iu 
this  case  the  intruding  air  may  possibly  play  the  same  part  as  that 
contained  in  the  air-chamber  of  a  force-pump — tending,  by  moderat- 
ing the  violence  of  the  jets,  and  by  equalizing  the  strains,  to  prevent 
rupture  of  the  apparatus.  Of  course  when  the  supply  of  liquid 
becomes  adequate,  and  the  strains  not  too  violent,  these  bubbles  will 
be  expelled  as  readily  as  they  entered. 

Here,  as  before,  let  me  add  the  conclusive  proof  furnished  by  a 
direct  experiment.  To  ascertain  the  amount  of  this  propulsive 
action,  I  took  from  the  same  tree,  a  Laurel,  two  equal  shoots,  and 
placing  them  in  the  same  dye,  subjected  them  to  conditions  that 
were  alike  hi  all  respects  save  that  of  motion :  while  one  remained 
at  rest,  the  other  was  bent  backwards  and  forwards,  now  by  switch- 
ing and  now  by  straining  with  the  fingers.  After  the  lapse  of  an 
hour,  I  found  that  the  dye  had  ascended  the  oscillating  shoot  three 
tunes  as  far  as  it  had  ascended  the  stationary  shoot — this  result 
being  an  average  from  several  trials.  Similar  trials  brought  out 
similar  effects  hi  other  structures.  The  various  petioles  and  herba- 
ceous shoots  experimented  upon  for  the  purpose  of  ascertaining  the 
amount  of  exudation  produced  by  transverse  strains,  showed  also 
the  amount  of  longitudinal  movement.  It  was  observable  that  the 
height  ascended  by  the  dye  was  in  all  cases  greater  where  there  had 
been  oscillation  than  where  there  had  bee*n  rest — the  difference, 
however,  being  much  less  marked  hi  succulent  structures  than  in 
woody  ones. 

It  need  scarcely  be  said  that  this  mechanical  action  is  not  here 
assigned  as  the  sole  cause  of  circulation,  but  as  a  cause  co-operating 
with  others,  and  helping  others  to  produce  effects  that  could  not 
otherwise  be  produced.  Trees  growing  hi  conservatories  afford  us 
abundant  proof  that  sap  is  raised  to  considerable  heights  by  other 
forces.  Though  it  is  notorious  that  trees  so  circumstanced  do  not 
thrive  unless,  through  open  sashes,  they  are  frequently  subject  to 
breezes  sufficient  to  make  their  parts  oscillate,  yet  there  is  evidently 
a  circulation  that  goes  on  without  mechanical  aid.  The  causes  of 
circulation  are  those  actions  only  which  disturb  the  liquid  equilibrium 
in  a  plant,  by  permanently  abstracting  water  or  sap  from  some  part 
of  it ;  and  of  these  the  first  is  the  absorption  of  materials  for  the  for- 
mation of  new  tissue  hi  growing  parts ;  the  second  is  the  loss  by 
evaporation,  mainly  through  adult  leaves  ;  and  the  third  is  the  loss  by 
extravasation,  through  compressed  vessels.  Only  so  far  as  it  pro- 
duces this  last,  can  mechanical  strain  be  regarded  as  truly  a  cause  of 
circulation.  All  the  other  actions  concerned  must  be  classed  as  aids 
to  circulation — as  facilitating  that  redistribution  of  liquid  that  con- 
tinually restores  the  equilibrium  continually  disturbed  ;  and  of  these, 


654 

capillary  action  may  be  named  as  the  first,  osmose  as  the  second,  and 
the  propulsive  effect  of  mechanical  strains  as  the  third.  The  first 
two  of  these  aids  are  doubtless  capable  by  themselves  of  producing  a 
large  part  of  the  observed  result — more  of  the  observed  result  than  is 
at  first  sight  manifest ;  for  there  is  an  important  indirect  effect  of 
osmotic  action  which  appears  to  be  overlooked.  Osmose  does  not 
aid  circulation  only  by  setting  up,  within  the  plant,  exchange  currents 
between  the  more  dense  and  the  less  dense  solutions  in  different  parts 
of  it ;  but  it  aids  circulation  much  more  by  producing  distention  of 
the  plant  as  a  whole.  In  consequence  of  the  average  contrast  in 
density  between  the  water  outside  of  the  plant  and  the  sap  inside  of  it, 
the  constant  tendency  is  for  the  plant  to  absorb  a  quantity  in  excess 
of  its  capacity,  and  so  to  produce  distention  and  erection  of  its 
tissues.  It  is  because  of  this  that  the  drooping  plant  raises  itself 
when  watered ;  for  capillary  action  alone  could  only  refill  its  tissues 
without  changing  their  attitudes.  And  it  is  because  of  this  that 
juicy  plants  with  collapsible  structures  bleed  so  rapidly  when  cut,  not 
only  from  the  cut  surface  of  the  rooted  part,  but  from  the  cut  sur- 
face of  the  detached  part — the  elastic  tissues  tending  to  press  out  the 
liquid  which  distends  them.  And  manifestly  if  osmose  serves  thus 
to  maintain  a  state  of  distention  throughout  a  plant,  it  indirectly  fur- 
thers circulation ;  since  immediately  evaporation  or  growth  at  any 
part,  by  abstracting  liquid  from  the  neighbouring  tissues,  begins  to 
diminish  the  liquid  pressure  within  such  tissues,  the  distended  struc- 
tures throughout  the  rest  of  the  plant  thrust  their  liquid  contents  to- 
wards the  place  of  diminished  pressure.  This,  indeed,  may  very  pos- 
sibly be  the  most  efficient  of  the  agencies  at  work.  Remembering 
how  great  is  the  distention  producible  by  osmotic  absorption — great 
enough  to  burst  a  bladder — it  is  clear  that  the  force  with  which  the 
distended  tissues  of  a  plant  urge  forward  the  sap  to  places  of  con- 
sumption, is  probably  very  great.  We  must  therefore  regard  the  aid 
which  mechanical  strains  give  as  being  one  of  several.  Oscillations 
help  directly  to  restore  any  disturbed  liquid  equilibrium ;  and  they 
also  help  indirectly,  by  facilitating  the  redistribution  caused  by  capil- 
lary action  and  the  process  just  described ;  but  in  the  absence  of 
oscillations  the  equilibrium  may  still  be  restored,  though  less  rapidly 
and  within  narrower  limits  of  distance. 

One  half  of  the  problem  of  the  circulation,  however,  has  been  left 
out  of  sight.  Thus  far  our  inquiry  has  been,  how  the  ascending  cur- 
rent of  sap  is  produced.  There  remains  the  rationale  of  the  descend- 
ing current.  What  forces  cause  it,  and  through  what  tissues  it  takes 
place,  are  questions  to  which  no  satisfactory  answers  have  been 
given.  That  the  descent  is  due  to  gravitation,  as  some  allege, 
is  difficult  to  conceive,  since,  as  gravitation  acts  equally  on  all 
liquid  columns  contained  in  the  stem,  it  is  not  easy  to  see 
why  it  should  produce  downward  movements  in  some  while  per- 


555 

mitting  upward  movements  in  others — unless,  indeed,  there  existed 
descending  tubes  too  wide  to  admit  of  much  capillary  action,  which 
there  do  not.  Moreover,  gravitation  is  clearly  inadequate  to  cause 
currents  towards  the  roots  out  of  branches  that  droop  to  the 
ground.  Here  the  gravitation  of  the  contained  liquid  columns 
must  nearly  balance  that  of  the  connected  columns  in  the  stem, 
leaving  no  appreciable  force  to  cause  motion.  Nor  does  there  seem 
much  probability  in  the  assumption  that  the  route  of  the  descending 
sap  is  through  the  cambium  layer,  since  experiments  on  the  absorp- 
tion of  dyes  prove  that  simple  cellular  tissue  is  a  very  bad  conductor 
of  liquids  :  their  movement  through  it  does  not  take  place  with  one- 
fiftieth  of  the  rapidity  with  which  it  takes  place  through  vessels.* 

Of  course  the  defence  for  these  hypotheses  is,  that  there  must  be  a 
downward  current,  which  must  have  a  course  and  a  cause ;  and  the 
very  natural  assumption  has  been  that  the  course  and  the  cause  must 
be  other  than  those  which  produce  the  ascending  current.  Never- 
theless there  is  an  alternative  supposition,  to  which  the  foregoing 
considerations  introduce  us.  It  is  quite  possible  for  the  same  vascular 
system  to  serve  as  a  channel  for  movement  in  opposite  directions  at 
different  times.  We  have  among  animals  well-known  cases  in 
which  the  blood-vessels  carry  a  current  first  in  one  direction  and 
then,  after  a  brief  pause,  in  the  reverse  direction.  And  there  seems 
an  ct  priori  probability  that,  lowly  organized  as  they  are,  plants  are 
more  likely  to  have  distributing  appliances  of  this  imperfect  kind  than 
to  have  two  sets  of  channels  for  two  simultaneous  currents.  If,  led 
by  this  suspicion,  we  inquire  whether  among  the  forces  which  unite 
to  produce  movements  of  sap,  there  are  any  variations  or  niter- 
missions  capable  of  determining  the  currents  in  different  directions, 
we  quickly  discover  that  there  are  such,  and  that  the  hypothesis  of 
an  alternating  motion  of  the  sap,  now  centrifugal  and  now  centri- 
petal, through  the  same  vessels,  has  good  warrant.  What  are  the 
several  forces  at  work?  First  may  be  set  down  that  tendency 
existing  in  every  part  of  a  plant  to  expand  into  its  typical  form,  and 
to  absorb  nutritive  liquids  hi  doing  this.  The  resulting  competition 

*  Some  exceptions  to  this  occur  in  plants  that  have  retrograded  in  the 
character  of  their  tissues  towards  the  simpler  vegetal  types.  Certain  very 
succulent  leaves,  such  as  those  of  Sempervivum,  in  which  the  cellular  tissue 
is  immensely  developed  in  comparison  with  the  vascular  tissue,  seem  to 
have  resumed  to  a  considerable  extent  what  we  must  regard  as  the  primitive 
form  of  vegetal  circulation — simple  absorption  from  cell  to  cell.  These, 
when  they  have  lost  much  of  their  water,  will  take  up  the  dye  to  some  dis- 
tance through  their  general  substance,  or  rather  through  its  interstices,  even 
neglecting  the  vessels.  At  other  times,  in  the  same  leaves,  the  vessels  will 
become  charged  while  comparatively  little  absorption  takes  place  through 
the  cellular  tissue.  Even  in  these  exceptional  cases,  however,  the  movement 
through  cellular  tissue  is  nothing  like  as  fast  as  the  movement  through 
vessels. 


558 

for  sap  will,  other  things  being  equal,  cause  currents  towards  tha 
most  rapidly-growing  parts — towards  unfolding  shoots  and  leaves, 
but  not  towards  adult  leaves.  Next  we  have  evaporation,  acting 
more  on  the  adult  leaves  than  on  those  which  are  in  the  bud, 
5r  but  partially  developed.  This  evaporation  is  both  regularly 
and  irregularly  intermittent.  Depending  chiefly  on  the  action 
of  the  sun,  it  is,  in  fine  weather,  greatly  checked  or  wholly 
arrested  every  evening ;  and  in  cloudy  weather  must  be  much* 
retarded  during  the  day.  Further,  every  hygrometric  variation, 
as  well  as  every  variation  in  the  movement  of  the  air,  must 
vary  the  evaporation.  This  chief  action,  therefore,  which,  by  con- 
tinually emptying  the  ends  of  the  capillary  tubes,  makes  upward 
currents  possible,  is  one  which  intermits  every  night,  and  every  day 
is  strong  or  feeble  as  circumstances  determine.  Then,  in  the  third 
place,  we  have  this  rude  pumping  process  above  described,  going  on 
with  greater  vigour  when  the  wind  is  violent,  and  with  less 
vigour  when  it  is  gentle  —  drawing  liquid  towards  different 
parts  according  to  their  degrees  of  oscillation,  and  from  diffe- 
rent parts  according  as  they  can  most  readily  furnish  it.  And 
now  let  us  ask  what  must  result  under  changing  conditions  from 
these  variously-conflicting  and  conspiring  forces.  When  a  warm 
sunshine,  causing  rapid  evaporation,  is  emptying  the  vessels  of  the 
leaves,  the  osmotic  and  capillary  actions  that  refill  them  will  be 
continually  aided  by  the  pumping  action  of  the  swaying  petioles, 
twigs,  and  branches,  provided  their  oscillations  are  moderate.  Under 
these  conditions  the  current  of  sap,  moving  in  the  direction  of  least 
resistance,  will  set  towards  the  leaves.  But  what  will  happen  when 
the  sun  sets?  There  is  now  nothing  to  determine  currents  either 
upwards  or  downwards,  except  the  relative  rates  of  growth  in  the 
parts  and  the  relative  demands  set  up  by  the  oscillations ;  and  the 
oscillations  acting  alone,  will  draw  sap  to  the  oscillating  parts  as 
much  from  above  as  from  below.  If  the  resistance  to  be  overcome 
by  a  current  setting  back  from  the  leaves  is  less  than  the  resistance 
to  be  overcome  by  a  current  setting  up  from  the  roots,  then  a 
current  will  set  back  from  the  leaves.  Now  it  is,  I  think,  tolerably 
manifest  that  in  the  swaying  twigs  and  minor  branches,  less  force 
will  be  required  to  overcome  the  inertia  of  the  short  columns  of 
liquid  between  them  and  the  leaves  than  to  overcome  the  inertia  of 
the  long  columns  between  them  and  the  roots.  Hence  during  the 
night,  as  also  at  other  tunes  when  evaporation  is  not  going  on,  the 
sap  will  be  drawn  out  of  the  leaves  into  the  adjacent  supporting 
parts  ;  and  their  nutrition  will  be  increased.  If  the  wind  is  strong 
enough  to  produce  a  swaying  of  the  thicker  branches,  the  back 
current  will  extend  to  them  also  ;  and  a  further  strengthening  will 
result  from  their  absorption  of  the  elaborated  sap.  And  when  the 
great  branches  and  the  stem  are  bent  backwards  and  forwards  by  a 


557 

pale,  they  too  will  share  in  the  nutrition.  It  may  at  first  sight  seem 
that  those  parts,  being  nearer  to  the  roots  than  to  the  leaves,  will 
draw  their  supplies  from  the  roots  only.  But  the  quantity  which  the 
roots  can  furnish  is  insufficient  to  meet  so  great  a  demand.  Under 
the  conditions  described,  the  exudation  of  sap  from  the  vessels  will 
be  very  great,  and  the  draught  of  liquid  required  to  refill  them,  not 
satisfied  by  that  which  the  root-fibres  can  take  in,  will  extend  to  the 
leaves.  Thus  sap  will  flow  to  the  several  parts  according  to  then* 
respective  degrees  of  activity — to  the  leaves  while  light  and  heat 
enable  them  to  discharge  then-  functions,  and  back  to  the  twigs, 
branches,  stem,  and  roots  when  these  become  active  and  the  leaves 
inactive,  or  when  their  activity  dominates  over  that  of  the  leaves. 
And  this  distribution  of  nutriment,  varying  with  the  varying 
activities  of  the  parts,  is  just  such  a  distribution  as  we  know  must  be 
required  to  keep  up  the  organic  balance. 

To  this  explanation  it  may  be  objected  that  it  does  not  account 
for  the  downward  current  of  sap  in  plants  that  are  sheltered.  The 
stem  and  roots  of  a  drawing-room  Geranium  display  a  thickening 
which  implies  that  nutritive  matters  have  descended  from  the  leaves, 
although  there  are  none  of  those  oscillations  by  which  the  sap  is  said 
to  be  drawn  downwards  as  well  as  upwards.  The  reply  is,  that  the 
stem  and  roots  tend  to  repeat  their  typical  structures,  and  that  tho 
absorption  of  sap  for  the  formation  of  their  respective  dense  tissues, 
is  here  the  force  which  determines  the  descent.  Indeed  it  must  be 
borne  in  mind  that  the  mechanical  strains  and  the  pumping  process 
which  they  keep  up,  as  well  as  the  distention  caused  by  osmose,  do 
not  in  themselves  produce  a  current  either  upwards  or  downwards : 
they  simply  help  to  move  the  sap  towards  that  place  where  there  is 
the  most  rapid  abstraction  of  it — the  place  towards  which  its  motion 
is  least  resisted.  Whether  there  is  oscillation  or  whether  there  is 
not,  the  physiological  demands  of  the  different  parts  of  the  plant 
determine  the  direction  of  the  cm-rent ;  and  all  which  the  oscillations 
and  the  distention  do  is  to  facilitate  the  supply  of  these  demands. 
Just  as  much,  therefore,  hi  a  plant  at  rest  as  in  a  plant  hi  motion, 
the  current  will  set  downwards  when  the  function  of  the  leaves  is 
arrested,  and  when  there  is  nothing  to  resist  that  abstraction  of  sap 
caused  by  the  tendency  of  the  stem-  and  root-tissues  to  assume  their 
typical  structures.  To  which  admission,  however,  it  must  be  added 
that  since  this  typical  structure  assumed,  though  imperfectly  assumed, 
by  the  hot-house  plant,  is  itself  interpretable  as  the  inherited  effect 
of  external  mechanical  actions  on  its  ancestors,  we  may  still  consider 
the  current  set  up  by  the  assumption  of  the  typical  structure  to  be 
indirectly  due  to  such  actions. 

Interesting  evidence  of  another  order  here  demands  notice.  In  the 
course  of  experiments  on  the  absorption  of  dyes  by  leaves,  it 
happened  that  in  making  sections  parallel  to  the  plane  of  a  leaf,  with 


558 

the  view  of  separating  its  middle  layer  containing  the  vessels,  I  came 
upon  some  structures  that  were  new  to  me.  These  structures,  where 
they  are  present,  form  the  terminations  of  the  vascular  system.  They 
are  masses  of  irregular  and  imperfectly  united  fibrous  cells,  such  as 
those  out  of  which  vessels  are  developed ;  and  they  are  sometimes 
slender,  sometimes  bulky — usually,  however,  being  more  or  less  club- 
shaped.  In  transverse  sections  of  leaves  their  distinctive  characters 
are  not  shown  :  they  are  taken  for  the  smaller  veins.  It  is  only  by 
carefully  slicing  away  the  surface  of  a  leaf  until  we  come  down 
to  that  part  which  contains  them,  that  we  get  any  idea  of  their 
nature.  Fig.  1  represents  a  specimen  taken  from  a  leaf  of 
Euphorbia  neriifolia.  Occupying  one  of  the  iaterspaces  of  the  ulti- 
mate venous  network,  it  consists  of  a  spirally-lined  duct  or  set  of 
ducts,  which  connects  with  the  neighbouring  vein  a  cluster  of  half- 
reticulated,  half-scalariform  cells.  These  cells  have  projections,  many 
of  them  tapering,  that  insert  themselves  into  the  adjacent  intercellular 
spaces,  thus  producing  an  extensive  surface  of  contact  between  the 
organ  and  the  imbedding  tissues.  A  further  trait  is,  that  the  en- 
sheathing  prosenchyma  is  either  but  little  developed  or  wholly  ab- 
sent ;  and  consequently  this  expanded  vascular  structure,  especially 
at  its  end,  comes  immediately  in  contact  with  the  tissues  concerned 
in  assimilation.  The  leaf  of  Euphorbia  neriifolia  is  a  very  fleshy 
one ;  and  in  it  these  organs  are  distributed  through  a  compact, 
though  watery,  cellular  mass.  But  in  any  leaf  of  the  ordinary  type 
which  possesses  them,  they  lie  in  the  network  parenchyma  composing 
its  lower  layer ;  and  wherever  they  occur  in  this  layer  its  cells  unite 
to  enclose  them.  This  arrangement  is  shown  in  fig.  2,  representing 
a  sample  from  the  Caoutchouc-leaf,  as  seen  with  the  upper  part  of 
its  envelope  removed ;  and  it  is  shown  still  more  clearly  in  a  sample 
from  the  leaf  of  Panax  Lessonii,  fig.  3.  Figures  4  and  5  represent, 
without  their  sheaths,  other  such  organs  from  the  leaves  of  P<tn«x 
Lessonii  and  Clusiaflava.  Some  relation  seems  to  exist  between 
their  forms  and  the  thicknesses  of  the  layers  in  which  they  lie. 
Certain  very  thick  leaves,  such  as  those  of  Clusia  flava,  have  them 
less  abundantly  distributed  than  is  usual,  but  more  massive.  Where 
the  parenchyma  is  developed  not  to  so  great  an  extreme,  though 
still  largely,  as  in  the  leaves  of  Holly,  Aucuba,  Camellia,  they  are 
not  so  bulky;  and  in  thinner  leaves,  like  those  of  Privet,  Elder, 
&c.,  they  become  longer  and  less  conspicuously  club-shaped.  Some 
adaptations  to  their  respective  positions  seem  implied  by  these  modi- 
fications ;  and  we  may  naturally  expect  that  in  many  thin  leaves 
these  free  ends,  becoming  still  narrower,  lose  the  distinctive  and 
suggestive  characters  possessed  by  those  shown  in  the  diagrams. 
Relations  of  this  kind  are  not  regular,  however.  In  various  other 
genera,  members  of  which  I  have  examined,  as  Rhus,  Viburnum, 
Griselinia,  Hrexia,  Botryodendron,  Percskia,  the  variations  in  the 


66» 

bulk  and  form  of  these  structures  are  not  directly  determined  by 
the  spaces  which  the  leaves  allow  :  obviously  there  are  other  modi- 
fying causes.  It  should  be  added  that  while  hese  expanded  free 
extremities  graduate  into  tapering  free  extremities,  not  differing 
from  ordinary  vessels,  they  also  pass  insensibly  into  the  ordinary  in- 
osculations. Occasionally,  along  with  numerous  free  endings,  there 
occur  loops  ;  and  from  such  loops  there  are  transitions  to  the  ulti- 
mate meshes  of  the  veins. 

These  organs  are  by  no  means  common  to  all  leaves.  In  many 
that  afford  ample  spaces  for  them  they  are  not  to  be  found.  So  far 
as  I  have  observed,  they  are  absent  from  the  thick  leaves  of  plants 
which  form  very  little  wood.  In  Sempervivum,  in  Echeveria,  in 
BryopkyUum,  they  do  not  appear  to  exist ;  and  I  have  been  unable 
to  discover  them  in  Kaianchoe  rotundifolia,  in  Kleinia  ante-euphorbium 
and  Jicoides,  in  the  several  species  of  Crassula,  and  in  other  succulent 
plants.  It  may  be  added  that  they  are  not  absolutely  confined  to 
leaves,  but  occur  in  stems  that  have  assumed  the  functions  of  leaves 
At  least  I  have  found,  in  the  green  parenchyma  of  Opuntia,  organs 
that  are  analogous  though  much  more  rudely  and  irregularly  formed. 
In  other  parts,  too,  that  have  usurped  the  leaf-function,  they  occur, 
as  in  the  phyllodes  of  the  Australian  Acacias.  These  have  them 
abundantly  developed ;  and  it  is  interesting  to  observe  that  here, 
where  the  two  vertically-placed  surfaces  of  the  flattened-out  petiole 
are  equally  adapted  to  the  assimilative  function,  there  exist  two 
layers  of  these  expanded  vascular  terminations,  one  applied  to  the 
inner  surface  of  each  layer  of  parenchyma. 

Considering  the  structures  and  positions  of  these  organs,  as  well 
as  the  natures  of  the  plants  possessing  them,  may  we  not  form  a 
shrewd  suspicion  respecting  their  function  ?  Is  it  not  probable  that 
they  facilitate  absorption  of  the  juices  carried  back  from  the  leaf  for 
the  nutrition  of  the  stem  and  roots  ?  They  are  admirably  adapted 
for  performing  this  office.  Their  component  fibrous  cells,  having 
angles  insinuated  between  the  cells  of  the  parenchyma,  are  shaped 
just  as  they  should  be  for  taking  up  its  contents  ;  and  the  absence 
of  sheathing  tissue  between  them  and  the  parenchyma  facilitates  the 
passage  of  the  elaborated  liquids.  Moreover  there  is  the  fact  that 
they  are  allied  to  organs  which  obviously  have  absorbent  functions. 
I  am  indebted  to  Dr.  Hooker  for  pointing  out  the  figures  of  two 
such  organs  in  the  "  Icones  Anatomies"  of  Link.  One  of  them  is 
from  the  end  of  a  dicotyledonous  root-fibre,  and  the  other  is  from 
the  prothallus  of  a  young  Fern.  In  each  case  a  cluster  of  fibrous 
cells,  seated  at  a  place  from  which  liquid  has  to  be  drawn,  is  con- 
nected by  vessels  with  the  parts  to  which  liquid  has  to  be  carried. 
There  can  scarcely  be  a  doubt,  then,  that  in  both  cases  absorption 
is  effected  through  them.  I  have  met  with  another  such  organ, 
more  elaborately  constructed,  but  evidently  adapted  to  the  same 


500 

office,  in  the  common  Turnip-root.  As  shown  by  the  end  view 
and  longitudinal  section  in  figs.  6  and  7,  this  organ  consists  of 
rings  of  fenestratcd  cells,  arranged  with  varying  degrees  of  regu- 
larity into  a  funnel,  ordinarily  having  its  apex  directed  towards  the 
central  mass  of  the  Turnip,  with  which  it  has,  in  some  cases  at  least, 
a  traceable  connexion  by  a  canal.  Presenting  as  it  does  an  external 
porous  surface  terminating  one  of  the  branches  of  the  vascular  sys- 
tem, each  of  these  organs  is  well  fitted  for  taking  up  with  rapidity 
the  nutriment  laid  by  in  the  Turnip-root,  and  used  by  the  plant 
when  it  sends  up  its  flower-stalk.  Nor  does  even  this  exhaust  the 
analogies.  The  cotyledons  of  the  young  bean,  experimented  upon 
as  before  described,  furnished  other  examples  of  such  structures, 
exactly  in  the  places  where,  if  they  are  absorbents,  we  might 
expect  to  find  them.  Amid  the  branchings  and  inosculations  of  the 
vascular  layer  running  through  the  mass  of  nutriment  deposited  in 
each  cotyledon,  there  are  conspicuous  free  terminations  that  are  club- 
shaped,  and  prove  to  be  composed,  like  those  in  leaves,  of  irregularly 
formed  and  clustered  fibrous  cells;  and  some  of  them,  diverging 
from  the  plane  of  the  vascular  layer,  dip  down  into  the  mass  of 
starch  and  albumen  which  the  young  plant  has  to  utilize,  and  which 
these  structures  can  have  no  other  function  but  to  take  up. 

Besides  being  so  well  fitted  for  absorption,  and  besides  being 
similar  to  organs  which  we  cannot  doubt  are  absorbents,  these 
vascular  terminations  in  leaves  afford  us  yet  another  evidence  of  their 
functions.  They  are  seated  in  a  tissue  so  arranged  as  specially  to 
facilitate  the  abstraction  of  liquid.  The  centripetal  movement  of  the 
sap  must  be  set  up  by  a  force  that  is  comparatively  feeble,  since,  the 
par'etes  of  the  ducts  being  porous,  air  will  enter  if  the  tension  on  the 
contained  columns  becomes  considerable.  Hence  it  is  needful  that 
the  exit  of  sap  from  the  leaves  should  meet  with  very  little  resistance. 
Now  were  it  not  for  an  adjustment  presently  to  be  described,  it  would 
meet  with  great  resistance,  notwithstanding  the  peculiar  fitness  of 
these  organs  to  take  't  in.  Liquid  cannot  be  drawn  out  of  any 
closed  cavity  without  producing  a  collapse  of  the  cavity's  sides ; 
and  if  .its  sides  are  not  readily  collapsible,  there  must  be  a  corre- 
sponding resistance  to  the  abstraction  of  liquid  from  it.  Clearly  the 
like  must  happen  if  the  liquid  is  to  be  drawn  out  of  a  tissue  which 
cannot  either  diminish  in  bulk  bodily  or  allow  its  components  indi- 
vidually to  diminish  in  bulk.  In  an  ordinary  leaf,  the  upper  layer  of 
parenchyma,  formed  as  it  is  of  closely-packed  cells  that  are  without 
interspaces,  and  are  everywhere  held  fast  within  their  framework  of 
vei'ns,  can  neither  contract  easily  as  a  mass,  nor  allow  its  separate 
cells  to  do  so.  Quite  otherwise  is  it  with  the  network-parenchyma 
below.  The  long  cells  of  this,  united  merely  by  their  ends  and 
hiving  their  flexible  sides  surrounded  by  air,  may  severally  have 
their  contents  considerably  increased  and  decreased  without  offering 


561 

appreciable  resistances;  and  the  network-tissue  which  they  form  will, 
nt  the  same  time,  be  capable  of  undergoing  slight  expansions  and  con- 
tractions of  its  thickness.  In  this  layer  occur  these  organs  that  are  so 
obviously  fitted  for  absorption.  Here  we  find  them  in  direct  communica- 
tion with  its  system  of  collapsible  cells.  The  probability  appears  to  be, 
that  when  the  current  sets  into  the  leaf,  it  passes  through  the  vessels 
and  their  sheaths  chiefly  into  the  upper  layer  of  cells  (this  upper 
layer  having  a  larger  surface  of  contact  with  the  veins  than  the 
lower  layer,  and  being  the  seat  of  more  active  processes)  ;  and  that 
the  juices  of  the  upper  layer,  enriched  by  the  assimilated  matters, 
pass  into  the  network  parenchyma,  which  serves  as  a  reservoir  from 
which  they  are  from  time  to  time  drawn  for  the  nutrition  of  the  rest 
of  the  plant,  when  the  actions  determine  the  downward  current. 
Should  it  be  asked  what  happens  where  the  absorbents,  instead  of 
being  inserted  in  a  network  parenchyma,  are,  as  in  the  leaves  of 
Euphorbia  neriifolia,  inserted  in  a  solid  parenchyma,  the  reply  is, 
that  such  a  parenchyma,  though  not  furnished  with  systematically 
arranged  air-chambers,  nevertheless  contains  air  hi  its  intercellular 
spaces ;  and  that  when  there  occurs  a  draught  upon  its  contents, 
the  expansion  of  this  air  and  the  entrance  of  more  from  without, 
quickly  supply  the  place  of  the  abstracted  liquid. 

If  then,  returning  to  the  general  argument,  we  conclude  that  these 
expanded  terminations  of  the  vascular  system  in  leaves  are  absorbent 
organs,  we  find  a  further  confirmation  of  the  views  set  forth  respect- 
ing the  alternating  movement  of  the  sap  along  the  same  channels. 
These  spongioles  of  the  leaves,  like  the  spongioles  of  the  roots,  being 
appliances  by  which  liquid  is  taken  up  to  be  carried  into  the  mass  of 
the  plant,  we  are  obliged  to  regard  the  vessels  that  end  in  these 
spongioles  of  the  leaves  as  being  the  channels  of  the  down  current 
whenever  it  is  produced.  If  the  elaborated  sap  is  abstracted  from 
the  leaves  by  these  absorbents,  then  we  have  no  alternative  but  to 
suppose  that,  having  entered  the  vascular  system,  the  elaborated  sap 
descends  through  it.  And  seeing  how,  by  the  help  of  these  special 
terminations,  it  becomes  possible  for  the  same  vessels  to  carry  back 
a  quality  of  sap  unlike  that  which  they  bring  up,  we  are  enabled  to 
understand  tolerably  well  how  this  rhythmical  movement  produces  a 
downward  transfer  of  materials  for  growth. 

The  several  lines  of  argument  may  now  be  brought  together ;  and 
nlong  with  them  may  be  woven  up  such  evidences  as  remain.  Let 
me  first  point  out  the  variety  of  questions  to  which  the  hypothesis 
supplies  answers. 

It  is  required  to  account  for  the  ascent  of  sap  to  a  height  beyona 
that  to  which  capillary  action  can  raise  it.  This  ascent  is  accounted 
for  by  the  propulsive  action  of  transverse  strains,  joined  with  that  of 
osmotic  distention.  A  cause  has  to  be  assigned  for  that  rise  of  sup 


f.63 

which,  in  the  spring,  while  yet  there  is  no  considerable  evaporation 
to  aid  it,  goes  on  with  a  power  which  capillarity  does  not  explain. 
The  co-operation  of  the  same  two  agencies  is  assignable  for  this  result 
also.*  The  circumstance  that  vessels  and  ducts  here  contain  sap  and 
there  contain  air,  and  at  the  same  place  contain  at  different  seasons 
now  air  and  now  sap  is  a  fact  calling  for  explanation.  An  explana- 
tion is  furnished  by  these  mechanical  actions  which  involve  the  en- 
trance or  expulsion  of  air  according  to  the  supply  of  liquid.  That 
vessels  and  ducts  which  were  originally  active  sap-carriers  go  com- 
pletely out  of  use,  and  have  their  function  discharged  by  other 
vessels  or  ducts,  is  an  anomaly  that  has  to  be  solved.  Again,  we 
are  supplied  with  a  solution :  these  deserted  vessels  and  ducts  are 
those  which,  by  the  formation  of  dense  tissue  outside  of  them,  be- 
come so  circumstanced  that  they  cannot  be  compressed  as  they 
originally  were.  A  channel  has  to  be  found  for  the  downward 
current  of  sap,  which,  on  any  other  hypothesis  than  the  foregoing, 
must  be  a  channel  separate  from  that  taken  by  the  upward 
current ;  and  yet  no  good  evidence  of  a  separate  channel  has'  been 
pointed  out.  Here,  however,  the  difficulty  disappears,  since  one 
channel  suffices  for  the  current  alternating  upwards  and  downwards 
according  to  the  conditions.  Moreover  there  has  to  be  found  a 
force  producing  or  facilitating  the  downward  current,  capable  even 
of  drawing  sap  out  of  drooping  branches  ;  and  no  such  force  is 
forthcoming.  The  hypothesis  set  forth  dispenses  with  this  necessity : 
under  the  recurring  change  of  conditions,  the  same  distention  and 
oscillation  which  before  raised  the  sap  to  the  places  of  consumption, 
now  bring  it  down  to  the  places  of  consumption.  A  physical 
process  has  to  be  pointed  out  by  which  the  material  that  forms 
dense  tissue  is  deposited  at  the  places  where  it  Ls  wanted,  rather 
than  at  other  places.  This  physical  process  the  hypothesis  in- 
dicates. It  is  requisite  to  find  an  explanation  of  the  fact  that, 
when  plants  ordinarily  swayed  about  by  the  wind  are  grown  indoors, 
the  formation  of  wood  is  so  much  diminished  that  they  become  abnor- 
mally slender.  Of  this  an  explanation  is  supplied.  Yet  a  further 

*  It  seems  probable,  however,  that  osmotic  distenfcion  is  here,  especially, 
the  more  important  of  the  two  factors.  The  rising  of  the  sap  in  spring  may 
indirectly  result,  like  the  sprouting  of  the  seed,  from  the  transformation  of 
starch  into  sugar.  During  germination,  this  change  of  an  oxy-hydro-carbou 
from  an  insoluble  into  a  soluble  form,  leads  to  rapid  endosmose ;  con- 
sequently to  great  distention  of  the  seed  ;  and  therefore  to  a  force  which 
thrusts  the  contained  liquids  into  the  plumule  and  radicle,  and  gives  them 
power  to  displace  the  soil  in  their  way  :  it  sets  up  an  active  internal  move- 
ment when  neither  evaporation  nor  the  change  which  light  produces  can  be 
operative.  And  similarly,  if,  in  the  spring,  the  starch  stored  up  in  the 
roots  of  a  tree  passes  into  the  form  of  sugar,  the  unusual  osmotic  absorption 
that  arises  will  cause  an  unusual  distention — a  distention  which,  being 
resisted  by  the  tough  bark  of  the  roots  and  stem,  will  result  in  a  powerful 
upward  thrust  of  the  contained  liquid. 


568 

Fact  to  be  interpreted  is,  that  in  the  same  individual  plant  homologous 
parts,  which,  according  to  the  type  of  the  plant,  shonld  be  equally 
woo'ly,  become  much  thicker  one  than  another  if  subject  to 
greater  mechanical  stress.  And  of  this  too  an  interpretation  is 
similarly  afforded. 

Now  the  sufficiency  of  the  assigned  actions  to  account  for  so  many 
phenomena  not  otherwise  explained,  would  be  strong  evidence  that 
the  rationale  is  the  true  one,  even  were  it  of  a  purely  hypothetical 
kind.  How  strong,  then,  becomes  the  reason  for  believing  it  tho 
true  one  when  we  remember  that  the  actions  alleged  demonstrably  go 
on  in  the  way  asserted.  They  are  ever  operating  before  our  eyes-, 
and  that  they  produce  the  effects  in  question  is  a  conclusion  dedu- 
cible  from  mechanical  principles,  a  conclusion  established  by  induction, 
and  a  conclusion  verified  by  experiment.  These  three  orders  of 
proof  may  be  briefly  summed  up  as  follows. 

That  plants  which  have  to  raise  themselves  above  the  earth's  sur- 
face, and  to  withstand  the  actions  of  the  wind,  must  have  a  power  ot 
developing  supporting  structure,  is  an  d,  priori  conclusion  which  may 
be  safely  drawn.  It  is  an  equally  safe  a  priori  conclusion,  that  it 
the  supporting  structure,  either  as  a  whole  or  in  any  of  its  parts,  has 
to  adapt  itself  to  the  particular  strains  which  the  individual  plant  is 
subject  to  by  its  particular  circumstances,  there  must  be  at  work 
some  process  by  which  the  strength  of  the  supporting  structure  i? 
everywhere  brought  into  equilibrium  with  the  forces  it  has  to  bear. 
Though  the  typical  distribution  of  supporting  structure  in  each  kind 
of  plant  may  be  explained  teleologically  by  those  whom  teleological 
explanations  satisfy ;  and  though  otherwise  this  typical  distribution 
may  be  ascribed  to  natural  selection  acting  apart  from  any  directly 
adaptive  process  ;  yet  it  is  manifest  that  those  departures  from  the 
typical  distribution  which  fit  the  parts  of  each  plant  to  their  special 
conditions  are  explicable  neither  teleologically  nor  by  natural  selec- 
tion. We  are,  therefore,  compelled  to  admit  that,  if  in  each  plant 
there  goes  on  a  balancing  of  the  particular  strains  by  the  particular 
strengths,  there  must  be  a  physical  or  physico-chemical  process  by 
which  the  adjustments  of  the  two  are  effected.  Meanwhile  we  are 
equally  compelled  to  admit,  a  priori,  that  the  mechanical  actions  to  be 
resisted,  themselves  affect  the  internal  tissues  in  such  ways  as  to  fur- 
ther the  increase  of  that  dense  substance  by  which  they  are  resisted. 
It  is  demonstrable  that  bending  the  petioles,  shoots,  and  sterns  must 
compress  the  vessels  beneath  their  surfaces,  and  increase  the  exuda- 
tion of  nutritive  matters  from  them,  and  must  do  this  actively  in  pro- 
portion as  the  bends  are  great  and  frequent ;  so  that  while,  on  the 
one  hand,  it  is  a  necessary  deduction  that,  if  the  parts  of  each  plant 
are  to  be  severally  strengthened  according  to  the  several  strains, 
there  must  be  some  direct  connexion  between  strains  and  strengths, 
it,  is,  on  the  other  hand,  a  necessary  deduction  from  mechanical  prin- 


ciplos  thut  the  strains  do  act  in  such  ways  as  to  aid  the  increase  of 
the  strengths.  How  a  like  correspondence  between  two  a  priori 
arguments  holds  in  the  case  of  the  circulation,  needs  not  to  be  shown 
in  detail.  It  will  suffice  to  remind  the  reader  that  while  the 
raising  of  sap  to  heights  beyond  the  limit  of  capillarity  implies  some 
force  to  effect  it,  we  have  in  the  osmotic  distention  and  the  intermit- 
tent compressions  caused  by  transverse  strains,  forces  which,  under 
the  conditions,  cannot  but  tend  to  effect  it ;  and  similarly  with  the  re- 
quirement for  a  downward  current,  and  the  production  of  a  down- 
ward current. 

Among  the  inductive  proofs  we  find  a  kindred  agreement.  Diffe- 
rent individuals  of  the  same  species,  and  different  parts  of  the  same 
individual,  do  strengthen  in  different  degrees  ;  and  there  is  a  clearly 
traceable  connexion  between  their  strengthenings  and  the  intermittent 
strains  they  are  exposed  to.  This  evidence,  derived  from  contrasts 
between  growths  on  the  same  plant  or  on  plants  of  the  same  type,  is 
enforced  by  evidence  derived  from  contrasts  between  plants  of  diffe- 
rent types.  The  deficiency  of  woody  tissue  which  we  see  in  plants 
called  succulent,  is  accompanied  by  a  bulkiness  of  the  parts  which 
prevents  any  considerable  oscillations  ;  and  this  character  is  also  habi- 
tually accompanied  by  a  dwarfed  growth.  When,  leaving  these  rela- 
tions as  displayed  externally,  we  examine  them  internally,  we  find 
the  facts  uniting  to  show,  by  their  agreements  and  differences,  that 
between  the  compression  of  the  sap-canals  and  the  production  oi 
wood  there  is  a  direct  relation.  We  have  the  facts,  that  in  each 
plant,  and  in  every  new  part  of  each  plant,  the  formation  of  sap- 
canals  precedes  the  formation  of  wood ;  that  the  deposit  of  wood} 
matter,  when  it  begins,  takes  place  around  these  sap-canals,  an! 
afterwards  around  the  new  sap-canals  successively  developed ;  thai 
this  formation  of  wood  around  the  sap-canals  takes  place  where  t\\t 
coats  of  the  canals  are  demonstrably  permeable,  and  that  the  amount 
of  wood-formation  is  proportionate  to  the  permeability.  And  then 
that  the  permeability  and  extravasation  of  sap  occur  wherever,  in 
the  individual  or  in  the  type,  there  are  intermittent  compressions,  is 
proved  alike  by  ordinary  cases  and  by  exceptional  cases.  In  the 
one  class  of  cases  we  see  that  the  deposit  of  wood  round  the  vessels 
begins  to  take  place  when  they  come  into  positions  that  subject 
ihem  to  intermittent  compressions.  •«  hile  it  ceases  when  they  become 
shielded  from  compressions.  And  in  the  other  class  of  cases,  where, 
from  the  beginning,  the  vessels  are  shielded  from  compression  by  sur- 
rounding fleshy  tissue,  there  is  a  permanent  absence  of  wood-forma- 
tion. 

To  which  complete  agreement  between  the  deductive  and  induc- 
tive inferences  has  to  be  added  the  direct  proof  supplied  by  experi- 
ments. It  is  put  beyond  doubt  by  experiment  that  the  liquids  ab- 
sorbed by  plants  are  distributed  to  their  different  purto  through  Iheii 


555 

vessels — at  first  by  the  spiral  or  allied  vessels  originally  developed, 
and  then  by  the  better-placed  ducts  formed  later.  By  experiment 
it  is  demonstrated  that  the  intermittent  compressions  caused  by  os- 
cillations urge  the  sap  along  the  vessels  and  ducts.  And  it  is  also  ex- 
perimentally proved  that  the  same  intermittent  compressions  produce 
exudation  of  sap  from  vessels  and  ducts  into  the  surrounding  tissue. 
That  the  processes  here  described,  acting  through  all  past  time, 
have  sufficed  of  themselves  to  develope  the  supporting  and  distribut- 
ing structures  of  plants,  is  not  alleged.  What  share  the  natural 
selection  of  variations  distinguished  as  spontaneous,  has  had  in  estab- 
lishing them,  is  a  question  which  remains  to  be  discussed.  Whether 
acting  alone  natural  selection  would  have  sufficed  to  evolve  these 
vascular  and  resisting  tissues,  I  do  not  profess  to  say.  That  it  has 
been  a  co-operating  cause,  I  take  to  be  self-evident :  it  must  all  along 
have  furthered  the  action  of  any  other  cause,  by  preserving  the  in- 
dividuals on  which  such  other  cause  had  acted  most  favourably. 
Seeing,  however,  the  conclusive  proof  which  we  have  that  another 
cause  has  been  in  action — certainly  on  individuals,  and,  in  all  proba- 
bility, by  inheritance  on  races — we  may  most  philosophically  ascribe 
the  genesis  of  these  internal  structures  to  this  cause,  and  regard 
natural  selection  as  having  here  played  the  part  of  an  accelerator 


EXPLANATION    OF    PLATE. 

Fig.  1.  Absorbent  organ  from  the  leaf  of  Euphorbia  neriifolia. 
The  cluster  of  fibrous  cells  forming  one  of  the  terminations  of  the 
vascular  system  is  here  imbedded  in  a  solid  parenchyma. 

Fig.  2.  A  structure  of  analogous  kind  from  the  leaf  of  Ficus 
clastica.  Here  the  expanded  terminations  of  the  vessels  are  im- 
bedded in  the  network  parenchyma,  the  cells  of  which  unite  to  form 
envelopes  for  them. 

Fig.  3.  Shows  on  a  larger  scale  one  of  these  absorbents  from 
the  leaf  of  Panax  Lessonii.  In  this  figure  is  clearly  seen  the  way  in 
which  the  cells  of  the  network  parenchyma  unite  into  a  closely- fitting 
case  for  the  spiral  cells. 

Fig.  4.  Represents  a  m  uch  more  Piassive  absorbent  from  the  same 
leaf,  the  surrounding  tissues  being  omitted. 

Fig.  5.  Similarly  represents,  without  its  sheath,  an  absorbent  from 
the  leaf  of  Chisiajiava. 

Fig.  6.  End  view  of  an  absorbent  organ  from  the  root  of  a 
Turnip.  It  is  taken  from  the  outermost  layer  of  vessels.  Its  funnel- 
shaped  interior  is  drawn  as  it  presents  itself  when  looked  at  from  the 
outside  of  this  layer,  its  narrow  end  being  directed  towards  tha 
centre  of  the  Turnip. 

Fig.  7  A  longitudinal  section  through  the  axis  of  another  such 
orgsn,  showing  its  annuli  of  reticulated  cells  when  cut  through.  The 
cellular  tissue  which  fills  the  interior  is  supposed  to  be  removed. 


566 


Fig.  8.  A  less-developed  absorbent,  showing  its  approximate  con- 
nexion with  a  duct.  In  their  simplest  forms,  these  structures  consist 
of  only  two  fenestrated  cells,  with  their  ends  bent  round  so  as  to 
m  ;et.  Such  types  occur  hi  the  central  mass  of  the  Turnip,  where 


the  vascular  system  is  relatively  imperfect.  Besides  the  compara- 
tively regular  forms  of  these  absorbents,  there  are  forms  composed 
of  amorphous  masses  of  fenestrated  cells.  It  should  be  added  that 
both  the  regular  and  irregular  kinds  are  very  variable  in  their  num- 
bers :  in  some  turnips  they  are  abundant,  and  in  others  scarcely  to  bo 
found.  Possibly  their  presence  depends  on  the  age  of  the  Turnip. 


APPENDIX  B, 


ON  THE  ORIGIN  OF  THE  VERTEBRATE  TYPE. 

[When  studying  the  development  of  the  vertebrate  skeleton,  there 
occurred  to  me  the  following  idea  respecting  the  jwssible  origin  of  the 
notochord.  1  was  eventually  led  to  omit  the  few  pages  of  Appendix  in 
u'ltich  I  had  expressed  this  idea,  because  it  was  unsupported  by  develop- 
mental evidence.  The  developmental  evidence  recently  discovered,  how- 
eccr,  has  led  Professor  Hacckel  and  others  to  analogous  views  respecting 
ihe  affiliation  of  the  Yertebrata  on  the  Molluscoida.  Having  fortu- 
nately preserved  a  proof  of  the  suppressed  pages,  I  am  able  now  to 
add  them.  With  the  omission  of  a  superfluous  paragraph,  they  arc 
reprinted  verbatim  from  this  proof,  which  dates  back  to  the  autumn  of 
18G5,  at  which  time  the  chapter  on  "The  Shapes  of  Vertebrate 
Skeletons"  was  written. — December,  1869.] 

The  general  argument  contained  in  Chap.  XVI.  of  Part  IV.,  I 
have  thought  it  undesirable  to  implicate  with  any  conception  more 
speculative  than  those  essential  to  it ;  and  to  avoid  so  implicating 
it,  I  transfer  to  this  place  an  hypothesis  respecting  the  derivation 
of  the  rudimentary  vertebrate  structure,  which  appears  to  me 
worth  considering. 

Among  those  molluscoid  animals  with  which  the  lowest  verte- 
brate animal  has  sundry  traits  in  common,  it  very  generally  happens 
that  while  the  adult  is  stationary  the  larva  is  locomotive.  The 
locomotion  of  the  larva  is  effected  by  the  undulations  of  a  tail.  In 
shape  and  movement  one  of  these  young  Ascidians  is  not  altogether 
unlike  a  Tadpole.  And  as  the  tail  of  the  Tadpole  disappears 
when  its  function  comes  to  be  fulfilled  by  limbs ;  so  the  Ascidiau 
larva's  tail  disappears  when  fixation  of  the  larva  renders  it  useless. 
This  disappearance  of  the  tail,  however,  is  not  without  exception. 
The  Appendicularia  is  an  Ascidian  which  retains  its  tail  through- 
out life ;  and  by  its  aid  continues  throughout  life  to  swim  about. 
Now  this  tail  of  the  Appendicttfaria  has  a  very  suggestive  structure. 
It  is  long,  tapering  to  a  point,  and  flattened.  From  end  to  end 
there  runs  a  mid-rib,  which  appears  to  be  an  imbedded  gelatinous 
rod,  not  unlike  a  notochord.  Extending  along  the  two  sides  of 


5G8  AITLNDIX. 

this  mid-rib,  arc  bundles  of  muscular  fibres;  and  its  top  bears  a 
gangliated  nervous  thread,  giving  off,  at  intervals,  branches  to  the 
muscular  fibres.  In  the  Appendiculaiia  this  tail,  which  is  inserted 
at  the  lower  part  of  the  back,  is  bent  forwards,  so  as  not  to  be 
adapted  for  propelling  the  body  of  the  animal  head  foremost ;  but 
the  homologous  tails  of  the  larval  Ascidians  are  directed  backwards, 
so  as  to  produce  forward  movement.  If  we  suppose  a  type  like  the 
Appendicularia  in  the  structure  and  insertion  of  its  permanent  tail, 
but  resembling  the  larval  forms  in  the  direction  of  its  tail,  it  is,  I 
think,  not  difficult  to  see  that  functional  adaptation  joined  with 
natural  selection,  might  readily  produce  a  type  approximating  to 
that  whose  origin  we  are  considering.  It  is  a  fair  assumption 
that  an  habitually  -  locomotive  creature  would  profit  by  in- 
creased power  of  locomotion.  This  granted,  it  follows  that 
such  further  development  of  the  tail-structures  as  might  arise 
from  enhanced  function,  and  such  better  distribution  of  them 
as  spontaneous  variation  might  from  time  to  time  initiate, 
would  be  perpetuated.  What  must  be  the  accompanying  changes? 
The  more  vigorous  action  of  such  an  appendage  implies  a  firmer 
insertion  into  the  body ;  and  this  would  be  effected  by  the  pro- 
longation forwards  of  the  central  axis  of  the  tail  into  the  creature's 
back.  As  fast  as  there  progressed  this  fusion  of  the  increasingly- 
powerful  tail  with  the  body,  the  body  would  begin  to  partake  of  its 
oscillations  ;  and  at  the  same  time  that  the  resistant  axis  of  the  tail 
advanced  along  the  dorsal  region,  its  accompanying  muscular  fibres 
would  spread  over  the  sides  of  the  body :  gradually  taking  such 
modified  directions  and  insertions  as  their  new  conditions  rendered 
most  advantageous.  Without  further  explanation,  those  who 
examine  drawings  of  the  structures  described,  will,  I  think, 
see  that  in  such  a  way  a  tail  homologous  with  that  of  the 
Appendicularia,  would  be  likely,  in  the  course  of  that  de- 
velopment required  for  its  greater  efficiency,  gradually  to 
encroach  on  the  body,  until  its  mid-rib  became  the  dorsal 
axis,  its  gangliated  nerve-thread  the  spinal  chord,  and  its 
muscular  fibres  the  myocommata.  Such  a  development  of  an 
appendage  into  a  dominant  part  of  the  organism,  though  at  first 
sight  a  startling  supposition,  is  not  without  plenty  of  parallels : 
instance  the  way  in  which  the  cerebral  ganglia,  originally  mere 
adjuncts  of  the  spinal  chord,  eventually  become  the  great  centres  of 
the  nervous  system  to  which  the  spinal  chord  is  quite  subordinate ; 
or  instance  the  way  in  which  the  limbs,  small  and  inconspicuous  in 
(islies,  become,  in  Man,  masses  which,  taken  together,  outweigh  the 
trunk.  It  may  be  added  that  these  familiar  cases  have  a  further 
appropriateness ;  for  they  exhibit  higher  degrees  of  that  came 
increasing  dominance  of  the  organs  of  external  relation,  which  the 
hypothesis  itself  implies. 


APPENDIX.  569 

Of  course,  if  the  rudimentary  vertebrate  apparatus  thus  grew 
into,  and  spread  over,  a  molluscoid  visceral  system,  the  formation 
of  the  notochord  under  the  action  of  alternating  transverse  strains, 
did  not  take  place  as  suggested  in  §  255;  but  it  does  not  therefore 
follow  that  its  differentiation  from  surrounding  tissues  was  not 
mechanically  initiated  in  the  way  described.  For  what  was  said  iu 
that  section  respecting  the  effects  of  lateral  bendings  of  the  body, 
equally  applies  to  lateral  bendings  of  the  tail ;  and  as  fast  as  the 
developing  tail  encroached  on  the  body,  the  body  would  become 
implicated  in  the  transverse  strains,  and  the  differentiation  would 
advance  forwards  under  the  influences  originally  alleged.  Obviously, 
too,  though  the  lateral  muscular  masses  would  in  this  case  have  a 
different  history  ;  yet  the  segmentation  of  them  would  be  eventually 
determined  by  the  assigned  causes.  For  as  fast  as  the  strata  of 
contractile  fibres,  developing  somewhat  in  advance  of  the  dorsal 
axis,  spread  along  the  sides,  they  would  come  under  the  influence 
of  the  alternate  flexions  ;  and  while,  by  survival  of  the  fittest,  their 
parts  became  adjusted  in  direction,  their  segmentation  would,  as 
before,  accompany  their  increasing  massiveness.  The  actions  and 
reactions  due  to  lateral  undulations  would  still,  therefore,  be  the 
causes  of  differentiation,  with  which  natural  selection  would  co- 
cerate. 


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Samoans. 


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